Index: wp-includes/taxonomy.php
===================================================================
--- wp-includes/taxonomy.php	(revision 16423)
+++ wp-includes/taxonomy.php	(working copy)
@@ -542,9 +542,6 @@
 				return ' AND 0 = 1';
 		}
 
-		if ( !in_array( $operator, array( 'IN', 'NOT IN' ) ) )
-			$operator = 'IN';
-
 		$taxonomies = "'" . implode( "', '", $taxonomies ) . "'";
 
 		$terms = array_unique( (array) $terms );
@@ -587,13 +584,12 @@
 
 			$i++;
 		}
-		else {
-			// NOT IN is very slow for some reason
-			$where .= " AND $primary_table.$primary_id_column IN (
+		elseif ( 'NOT IN' == $operator ) {
+			$where .= " AND $primary_table.$primary_id_column NOT IN (
 				SELECT object_id 
 				FROM $wpdb->term_relationships 
-				WHERE term_taxonomy_id $operator ($terms) 
-			)"; 		
+				WHERE term_taxonomy_id IN ($terms)
+			)";
 		}
 	}