Index: includes/common.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/common.inc,v
retrieving revision 1.878
diff -u -p -r1.878 common.inc
--- includes/common.inc	22 Apr 2009 09:45:02 -0000	1.878
+++ includes/common.inc	23 Apr 2009 10:16:09 -0000
@@ -3248,8 +3248,7 @@ function drupal_alter($type, &$data) {
 function drupal_get_page($content = NULL) {
   // Initialize page array with defaults. @see hook_elements() - 'page' element.
   $page = element_info('page');
-  $page['content'] = is_array($content) ? $content : array('main' => array('#markup' => $content));
-
+  system_set_content_block($content);
   return $page;
 }
 
Index: includes/theme.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/theme.inc,v
retrieving revision 1.475
diff -u -p -r1.475 theme.inc
--- includes/theme.inc	20 Apr 2009 21:28:13 -0000	1.475
+++ includes/theme.inc	23 Apr 2009 10:16:09 -0000
@@ -2053,6 +2053,11 @@ function template_preprocess_block(&$var
   $variables['block_zebra'] = ($block_counter[$variables['block']->region] % 2) ? 'odd' : 'even';
   $variables['block_id'] = $block_counter[$variables['block']->region]++;
 
+  if (is_array($variables['block']->content)) {
+    // Render the block contents if it is not already rendered.
+    $variables['block']->content = drupal_render($variables['block']->content);
+  }
+
   $variables['template_files'][] = 'block-' . $variables['block']->region;
   $variables['template_files'][] = 'block-' . $variables['block']->module;
   $variables['template_files'][] = 'block-' . $variables['block']->module . '-' . $variables['block']->delta;
Index: modules/block/block.admin.inc
===================================================================
RCS file: /cvs/drupal/drupal/modules/block/block.admin.inc,v
retrieving revision 1.35
diff -u -p -r1.35 block.admin.inc
--- modules/block/block.admin.inc	11 Feb 2009 03:38:46 -0000	1.35
+++ modules/block/block.admin.inc	23 Apr 2009 10:16:09 -0000
@@ -36,6 +36,9 @@ function block_admin_display_form(&$form
 
   $block_regions = system_region_list($theme_key) + array(BLOCK_REGION_NONE => '<' . t('none') . '>');
 
+  // Use form submission data to show values if showing the form again.
+  $blocks = (!empty($form_state['values']) ? $form_state['values'] : $blocks);
+  
   // Weights range from -delta to +delta, so delta should be at least half
   // of the amount of blocks present. This makes sure all blocks in the same
   // region get an unique weight.
@@ -95,6 +98,19 @@ function block_admin_display_form(&$form
 }
 
 /**
+ * Validate main blocks administration form submissions.
+ */
+function block_admin_display_form_validate($form, &$form_state) {
+  foreach ($form_state['values'] as $key => $block) {
+    if (($block['module'] == 'system') && ($block['delta'] == 'main') && ($block['region'] == BLOCK_REGION_NONE)) {
+      form_set_error($key, t('Disabling the main page content block makes your system unusable. You should always enable it to display in a region.'));
+      // Suggest to put it back into the content region.
+      $form_state['values'][$key]['region'] = 'content';
+    }
+  }
+}
+
+/**
  * Process main blocks administration form submissions.
  */
 function block_admin_display_form_submit($form, &$form_state) {
Index: modules/system/system.admin.inc
===================================================================
RCS file: /cvs/drupal/drupal/modules/system/system.admin.inc,v
retrieving revision 1.136
diff -u -p -r1.136 system.admin.inc
--- modules/system/system.admin.inc	21 Apr 2009 09:31:31 -0000	1.136
+++ modules/system/system.admin.inc	23 Apr 2009 10:16:09 -0000
@@ -179,7 +179,9 @@ function system_themes_form() {
     }
     else {
       // Ensure this theme is compatible with this version of core.
-      if (!isset($theme->info['core']) || $theme->info['core'] != DRUPAL_CORE_COMPATIBILITY) {
+      // Require the 'content' region to make sure the main page 
+      // content has a common place in all themes.
+      if (!isset($theme->info['core']) || ($theme->info['core'] != DRUPAL_CORE_COMPATIBILITY) || (!isset($theme->info['regions']['content']))) {
         $incompatible_core[] = $theme->name;
       }
       if (version_compare(phpversion(), $theme->info['php']) < 0) {
Index: modules/system/system.install
===================================================================
RCS file: /cvs/drupal/drupal/modules/system/system.install,v
retrieving revision 1.316
diff -u -p -r1.316 system.install
--- modules/system/system.install	20 Apr 2009 02:23:16 -0000	1.316
+++ modules/system/system.install	23 Apr 2009 10:16:09 -0000
@@ -3244,6 +3244,25 @@ function system_update_7020() {
 }
 
 /**
+ * Enable the main content block in all themes.
+ */
+function system_update_7021() {
+  $ret = array();
+  $themes = list_themes();
+  foreach ($themes as $theme) {
+    if (db_result(db_query("SELECT COUNT(*) FROM {block} WHERE theme = '%s'", $theme->name))) {
+      // Add new block to all themes which had at least one block, even disabled ones.
+      // For those, which do not have blocks yet, they were not yet enabled, so we
+      // should not add this one, because that would prevent 
+      // block_initialize_theme_blocks() to also copy over the other blocks, when the
+      // theme is enabled later.
+      db_query("INSERT INTO {block} (module, delta, theme, status, weight, region, pages, cache) VALUES ('%s', '%s', '%s', %d, %d, '%s', '%s', %d)", 'system', 'main', $theme->name, 1, 0, 'content', '', -1);
+    }
+  }
+  return $ret;
+}
+
+/**
  * @} End of "defgroup updates-6.x-to-7.x"
  * The next series of updates should start at 8000.
  */
Index: modules/system/system.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/system/system.module,v
retrieving revision 1.682
diff -u -p -r1.682 system.module
--- modules/system/system.module	22 Apr 2009 09:45:03 -0000	1.682
+++ modules/system/system.module	23 Apr 2009 10:16:10 -0000
@@ -889,6 +889,11 @@ function system_user_timezone(&$edit, &$
  * Implementation of hook_block_list().
  */
 function system_block_list() {
+  $blocks['main'] = array(
+    'info' => t('Main page content'),
+     // Cached elsewhere.
+    'cache' => BLOCK_NO_CACHE,
+  );
   $blocks['powered-by'] = array(
     'info' => t('Powered by Drupal'),
     'weight' => '10',
@@ -954,6 +959,10 @@ function system_block_save($delta = '', 
 function system_block_view($delta = '') {
   $block = array();
   switch ($delta) {
+    case 'main':
+      $block['subject'] = NULL;
+      $block['content'] = system_set_content_block();
+      return $block;
     case 'powered-by':
       $image_path = 'misc/' . variable_get('drupal_badge_color', 'powered-blue') . '-' . variable_get('drupal_badge_size', '80x15') . '.png';
       // Don't display a title.
@@ -973,6 +982,23 @@ function system_block_view($delta = '') 
 }
 
 /**
+ * Memory for the page content so that it can be put into a region as a block.
+ *
+ * Given the nature of the Drupal page handling, this will be called once with
+ * a string or array. We store that and return it later as the block is being
+ * displayed.
+ */
+function system_set_content_block($content = NULL) {
+  $content_block = &drupal_static(__FUNCTION__, NULL);
+  if (!empty($content)) {
+    $content_block = (is_array($content) ? $content : array('main' => array('#markup' => $content)));
+  }
+  else {
+    return $content_block;
+  }
+}
+
+/**
  * Provide a single block on the administration overview page.
  *
  * @param $item
Index: profiles/default/default.profile
===================================================================
RCS file: /cvs/drupal/drupal/profiles/default/default.profile,v
retrieving revision 1.39
diff -u -p -r1.39 default.profile
--- profiles/default/default.profile	20 Mar 2009 19:18:11 -0000	1.39
+++ profiles/default/default.profile	23 Apr 2009 10:16:10 -0000
@@ -91,7 +91,8 @@ function default_profile_task_list() {
  */
 function default_profile_tasks(&$task, $url) {
   
-  // Enable 4 standard blocks.
+  // Enable 5 standard blocks.
+  db_query("INSERT INTO {block} (module, delta, theme, status, weight, region, pages, cache) VALUES ('%s', '%s', '%s', %d, %d, '%s', '%s', %d)", 'system', 'main', 'garland', 1, 0, 'content', '', -1);
   db_query("INSERT INTO {block} (module, delta, theme, status, weight, region, pages, cache) VALUES ('%s', '%s', '%s', %d, %d, '%s', '%s', %d)", 'user', 'login', 'garland', 1, 0, 'left', '', -1);
   db_query("INSERT INTO {block} (module, delta, theme, status, weight, region, pages, cache) VALUES ('%s', '%s', '%s', %d, %d, '%s', '%s', %d)", 'system', 'navigation', 'garland', 1, 0, 'left', '', -1);
   db_query("INSERT INTO {block} (module, delta, theme, status, weight, region, pages, cache) VALUES ('%s', '%s', '%s', %d, %d, '%s', '%s', %d)", 'system', 'management', 'garland', 1, 1, 'left', '', -1);
Index: profiles/expert/expert.profile
===================================================================
RCS file: /cvs/drupal/drupal/profiles/expert/expert.profile,v
retrieving revision 1.5
diff -u -p -r1.5 expert.profile
--- profiles/expert/expert.profile	20 Mar 2009 19:18:11 -0000	1.5
+++ profiles/expert/expert.profile	23 Apr 2009 10:16:10 -0000
@@ -42,7 +42,8 @@ function expert_profile_task_list() {
  * Perform any final installation tasks for this profile.
  */
 function expert_profile_tasks(&$task, $url) {
-  // Enable 3 standard blocks.
+  // Enable 4 standard blocks.
+  db_query("INSERT INTO {block} (module, delta, theme, status, weight, region, pages, cache) VALUES ('%s', '%s', '%s', %d, %d, '%s', '%s', %d)", 'system', 'main', 'garland', 1, 0, 'content', '', -1);
   db_query("INSERT INTO {block} (module, delta, theme, status, weight, region, pages, cache) VALUES ('%s', '%s', '%s', %d, %d, '%s', '%s', %d)", 'user', 'login', 'garland', 1, 0, 'left', '', -1);
   db_query("INSERT INTO {block} (module, delta, theme, status, weight, region, pages, cache) VALUES ('%s', '%s', '%s', %d, %d, '%s', '%s', %d)", 'system', 'navigation', 'garland', 1, 0, 'left', '', -1);
   db_query("INSERT INTO {block} (module, delta, theme, status, weight, region, pages, cache) VALUES ('%s', '%s', '%s', %d, %d, '%s', '%s', %d)", 'system', 'management', 'garland', 1, 1, 'left', '', -1);