Index: wp-includes/l10n.php
===================================================================
--- wp-includes/l10n.php	(revision 24119)
+++ wp-includes/l10n.php	(working copy)
@@ -409,22 +409,25 @@
  * @since 1.5.0
  *
  * @param string $domain Unique identifier for retrieving translated strings
- * @param string $abs_rel_path Optional. Relative path to ABSPATH of a folder,
- * 	where the .mo file resides. Deprecated, but still functional until 2.7
- * @param string $plugin_rel_path Optional. Relative path to WP_PLUGIN_DIR. This is the preferred argument to use. It takes precedence over $abs_rel_path
+ * @param string $deprecated Deprecated.
+ * @param string $plugin_rel_path Optional. Relative path to WP_PLUGIN_DIR.
+ * @return bool True on success, false on failure
  */
-function load_plugin_textdomain( $domain, $abs_rel_path = false, $plugin_rel_path = false ) {
+function load_plugin_textdomain( $domain, $deprecated = false, $plugin_rel_path = false ) {
 	$locale = apply_filters( 'plugin_locale', get_locale(), $domain );
 
 	if ( false !== $plugin_rel_path	) {
 		$path = WP_PLUGIN_DIR . '/' . trim( $plugin_rel_path, '/' );
-	} else if ( false !== $abs_rel_path ) {
+	} else if ( false !== $deprecated ) {
 		_deprecated_argument( __FUNCTION__, '2.7' );
-		$path = ABSPATH . trim( $abs_rel_path, '/' );
+		$path = ABSPATH . trim( $deprecated, '/' );
 	} else {
 		$path = WP_PLUGIN_DIR;
 	}
 
+	if ( !is_readable( $path ) )
+		$path = WPMU_PLUGIN_DIR . '/' . trim( $plugin_rel_path, '/' );
+
 	$mofile = $path . '/'. $domain . '-' . $locale . '.mo';
 	return load_textdomain( $domain, $mofile );
 }