=== modified file 'modules/field_ui/field_ui.admin.inc'
--- modules/field_ui/field_ui.admin.inc	2009-08-29 08:00:03 +0000
+++ modules/field_ui/field_ui.admin.inc	2009-09-02 06:15:21 +0000
@@ -1007,6 +1007,7 @@
 
   $field = field_info_field($instance['field_name']);
   $form['#field'] = $field;
+  $form['#instance'] = $instance;
 
   if (!empty($field['locked'])) {
     $form['locked'] = array(

=== modified file 'modules/user/user.module'
--- modules/user/user.module	2009-08-31 18:00:03 +0000
+++ modules/user/user.module	2009-09-02 11:49:28 +0000
@@ -3065,6 +3065,17 @@
   if (count(element_children($form)) == 1) {
     $form['account']['#type'] = 'markup';
   }
+  
+  // Add user fields to the registration form.
+  field_attach_form('user', $null, $form, $form_state);
+  if (isset($form['#fields']) && is_array($form['#fields'])) {
+    // Don't display fields that aren't set to show on the registration pages.
+    foreach ($form['#fields'] as $field) {
+      if (!$field['instance']['settings']['user_register']) {
+        $form[$field['field']['field_name']]['#access'] = FALSE;
+      }
+    }
+  }
 
   $form['submit'] = array('#type' => 'submit', '#value' => t('Create new account'), '#weight' => 30);
   $form['#validate'][] = 'user_register_validate';
@@ -3130,3 +3141,27 @@
       ->execute();
   }
 }
+
+/**
+ * Implementation of hook_field_info_alter().
+ */
+function user_field_info_alter(&$info) {
+  foreach ($info as $field_type_name => $field_type) {
+    $info[$field_type_name]['instance_settings']['user_register'] = FALSE;
+  }
+}
+
+/**
+ * Add a checkbox to the field settings form to add the user
+ */
+function user_form_field_ui_field_edit_form_alter(&$form, &$form_state) {
+  $instance= $form['#instance'];
+  
+  if ($instance['bundle'] == 'user') {
+    $form['instance']['settings']['user_register'] = array(
+      '#type' => 'checkbox',
+      '#title' => t('Display this field on the registration form.'),
+      '#default_value' => $instance['settings']['user_register'],
+    );
+  }
+}