Index: modules/forum/forum.install
===================================================================
RCS file: /cvs/drupal/drupal/modules/forum/forum.install,v
retrieving revision 1.15
diff -u -p -r1.15 forum.install
--- modules/forum/forum.install	20 Dec 2007 21:50:11 -0000	1.15
+++ modules/forum/forum.install	31 Dec 2007 15:31:45 -0000
@@ -12,12 +12,17 @@ function forum_install() {
 }
 
 function forum_enable() {
-  // Create the forum vocabulary if it does not exist. Assign the vocabulary
-  // a low weight so it will appear first in forum topic create and edit
-  // forms.
-  $vid = variable_get('forum_nav_vocabulary', 0);
-  $vocabularies = taxonomy_get_vocabularies();
-  if (!isset($vocabularies[$vid])) {
+  if ($vocabulary = taxonomy_vocabulary_load(variable_get('forum_nav_vocabulary', 0))) {
+    // Existing install. Add back forum node type, if the forums
+    // vocabulary still exists. Keep all other node types intact there.
+    $vocabulary = (array) $vocabulary;
+    $vocabulary['nodes']['forum'] = 1;
+    taxonomy_save_vocabulary($vocabulary);
+  }
+  else {
+    // Create the forum vocabulary if it does not exist. Assign the vocabulary
+    // a low weight so it will appear first in forum topic create and edit
+    // forms.
     $vocabulary = array(
       'name' => t('Forums'),
       'multiple' => 0,
@@ -123,4 +128,4 @@ function forum_update_6000() {
   }
 
   return $ret;
-}
\ No newline at end of file
+}
Index: modules/taxonomy/taxonomy.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/taxonomy/taxonomy.module,v
retrieving revision 1.411
diff -u -p -r1.411 taxonomy.module
--- modules/taxonomy/taxonomy.module	22 Dec 2007 23:24:25 -0000	1.411
+++ modules/taxonomy/taxonomy.module	31 Dec 2007 15:31:46 -0000
@@ -967,14 +967,18 @@ function taxonomy_get_term_by_name($name
  * @param $vid
  *   The vocabulary's ID
  *
- * @return Object
- *   The vocabulary object with all of its metadata.
+ * @return
+ *   The vocabulary object with all of its metadata, if exists, NULL otherwise.
  *   Results are statically cached.
  */
 function taxonomy_vocabulary_load($vid) {
   static $vocabularies = array();
 
-  if (!array_key_exists($vid, $vocabularies)) {
+  if (!isset($vocabularies[$vid])) {
+    // Initialize so if this vocabulary does not exist, we have
+    // that cached, and we will not try to load this later.
+    $vocabularies[$vid] = FALSE;
+    // Try to load the data and fill up the object.
     $result = db_query('SELECT v.*, n.type FROM {vocabulary} v LEFT JOIN {vocabulary_node_types} n ON v.vid = n.vid WHERE v.vid = %d', $vid);
     $node_types = array();
     while ($voc = db_fetch_object($result)) {
@@ -986,8 +990,9 @@ function taxonomy_vocabulary_load($vid) 
       $vocabularies[$vid] = $voc;
     }
   }
-
-  return $vocabularies[$vid];
+  
+  // Return NULL if this vocabulary does not exist.
+  return !empty($vocabularies[$vid]) ? $vocabularies[$vid] : NULL;
 }
 
 /**