Index: wp-admin/includes/ms.php
===================================================================
--- wp-admin/includes/ms.php	(revision 21224)
+++ wp-admin/includes/ms.php	(working copy)
@@ -279,13 +279,13 @@
  * @return bool True if space is available, false otherwise.
  */
 function is_upload_space_available() {
-	if ( get_site_option( 'upload_space_check_disabled' ) )
-		return true;
+
+	$is_upload_space_available = true;
 
 	if ( !( $space_allowed = get_upload_space_available() ) )
-		return false;
+		$is_upload_space_available = false;
 
-	return true;
+	return apply_filters( 'is_upload_space_available', $is_upload_space_available );
 }
 
 /**