Index: src/wp-includes/comment.php
===================================================================
--- src/wp-includes/comment.php	(revision 27164)
+++ src/wp-includes/comment.php	(working copy)
@@ -648,7 +648,7 @@
  * @param string $meta_key Metadata key.
  * @param mixed $meta_value Metadata value.
  * @param mixed $prev_value Optional. Previous value to check before removing.
- * @return bool True on success, false on failure.
+ * @return int|bool Meta ID if the key didn't exist, true on successful update, false on failure.
  */
 function update_comment_meta($comment_id, $meta_key, $meta_value, $prev_value = '') {
 	return update_metadata('comment', $comment_id, $meta_key, $meta_value, $prev_value);
Index: src/wp-includes/meta.php
===================================================================
--- src/wp-includes/meta.php	(revision 27164)
+++ src/wp-includes/meta.php	(working copy)
@@ -26,7 +26,7 @@
  * @param bool $unique Optional, default is false. Whether the specified metadata key should be
  * 		unique for the object. If true, and the object already has a value for the specified
  * 		metadata key, no change will be made
- * @return int|bool The meta ID on successful update, false on failure.
+ * @return int|bool The meta ID on success, false on failure.
  */
 function add_metadata($meta_type, $object_id, $meta_key, $meta_value, $unique = false) {
 	if ( !$meta_type || !$meta_key )
@@ -96,7 +96,7 @@
  * @param mixed $meta_value Metadata value. Must be serializable if non-scalar.
  * @param mixed $prev_value Optional. If specified, only update existing metadata entries with
  * 		the specified value. Otherwise, update all entries.
- * @return bool True on successful update, false on failure.
+ * @return int|bool Meta ID if the key didn't exist, true on successful update, false on failure.
  */
 function update_metadata($meta_type, $object_id, $meta_key, $meta_value, $prev_value = '') {
 	if ( !$meta_type || !$meta_key )
Index: src/wp-includes/post.php
===================================================================
--- src/wp-includes/post.php	(revision 27164)
+++ src/wp-includes/post.php	(working copy)
@@ -1789,7 +1789,7 @@
  * @param string $meta_key Metadata key.
  * @param mixed $meta_value Metadata value. Must be serializable if non-scalar.
  * @param mixed $prev_value Optional. Previous value to check before removing.
- * @return bool True on success, false on failure.
+ * @return int|bool Meta ID if the key didn't exist, true on successful update, false on failure.
  */
 function update_post_meta($post_id, $meta_key, $meta_value, $prev_value = '') {
 	// make sure meta is added to the post, not a revision
Index: src/wp-includes/user.php
===================================================================
--- src/wp-includes/user.php	(revision 27164)
+++ src/wp-includes/user.php	(working copy)
@@ -988,7 +988,7 @@
  * @param string $meta_key Metadata key.
  * @param mixed $meta_value Metadata value.
  * @param mixed $prev_value Optional. Previous value to check before removing.
- * @return bool True on success, false on failure.
+ * @return int|bool Meta ID if the key didn't exist, true on successful update, false on failure.
  */
 function update_user_meta($user_id, $meta_key, $meta_value, $prev_value = '') {
 	return update_metadata('user', $user_id, $meta_key, $meta_value, $prev_value);