Index: wp-includes/ms-functions.php
===================================================================
--- wp-includes/ms-functions.php	(revision 23542)
+++ wp-includes/ms-functions.php	(working copy)
@@ -1930,9 +1930,9 @@
 		$space_allowed = get_site_option( 'blog_upload_space' );
 
 	if ( empty( $space_allowed ) || ! is_numeric( $space_allowed ) )
-		$space_allowed = 50;
+		$space_allowed = apply_filters( 'get_space_allowed_default', 50 );
 
-	return $space_allowed;
+	return apply_filters( 'get_space_allowed', $space_allowed );
 }
 
 /**