Index: wp-includes/query.php
===================================================================
--- wp-includes/query.php	(revision 12726)
+++ wp-includes/query.php	(working copy)
@@ -221,24 +221,53 @@
 }
 
 /**
- * Whether the current page query has the given taxonomy slug or contains taxonomy.
+ * Whether the current query is for the given taxonomy and/or term.
  *
+ * If no taxonomy argument is set, returns true if any taxonomy is queried.
+ * If the taxonomy argument is passed but no term argument, returns true 
+ *    if the taxonomy or taxonomies in the argument are being queried.
+ * If both taxonomy and term arguments are passed, returns true 
+ *    if the current query is for a term contained in the terms argument
+ *    which has a taxonomy contained in the taxonomy argument.
+ *
  * @since 2.5.0
  * @uses $wp_query
  *
- * @param string|array $slug Optional. Slug or slugs to check in current query.
+ * @param string|array $taxonomy Optional. Slug or slugs to check in current query.
+ * @param int|array|string $term. Optional. The term's ID, array of term IDs, or term slug.
  * @return bool
  */
-function is_tax( $slug = '' ) {
-	global $wp_query;
+function is_tax( $taxonomy = '', $term = '' ) {
+	global $wp_query, $wp_taxonomies;
 
-	if ( !$wp_query->is_tax )
-		return false;
+	$queried_object = $wp_query->get_queried_object();
+	$tax_array = (array) $taxonomy;
+	$term_array = (array) $term;
 
-	if ( empty($slug) )
+	if ( 
+		( $wp_query->is_tax || $wp_query->is_category || $wp_query->is_tag ) &&
+		( 
+			empty( $taxonomy ) || 
+			count( array_intersect($tax_array, array_keys($wp_taxonomies)) ) && 
+			(
+				isset($queried_object->taxonomy) &&
+				in_array($queried_object->taxonomy, $tax_array) &&
+				(
+					empty( $term ) ||
+					(
+						isset($queried_object->term_id) && 
+						count(array_intersect(
+							array($queried_object->term_id, $queried_object->name, $queried_object->slug), 
+							$term_array
+						))
+					)
+				)
+			)
+		)
+	) {
 		return true;
-
-	return in_array( get_query_var('taxonomy'), (array) $slug );
+	}
+	return false;
 }
 
 /**