Index: wp-includes/post.php
===================================================================
--- wp-includes/post.php	(revision 14433)
+++ wp-includes/post.php	(working copy)
@@ -2096,6 +2096,10 @@
 			$post_category = array();
 	}
 
+	// Make sure the category is set in [0] if there is only one
+	if ( 1 == count($post_category) && empty( $post_category[0] ) )
+		$post_category[0] = array_shift($post_category);
+
 	if ( empty($post_author) )
 		$post_author = $user_ID;