Index: wp-includes/post.php
===================================================================
--- wp-includes/post.php	(revision 15836)
+++ wp-includes/post.php	(working copy)
@@ -1183,7 +1183,7 @@
 	$r = wp_parse_args( $args, $defaults );
 	if ( empty( $r['post_status'] ) )
 		$r['post_status'] = ( 'attachment' == $r['post_type'] ) ? 'inherit' : 'publish';
-	if ( ! empty($r['numberposts']) )
+	if ( ! empty($r['numberposts']) && empty($r['posts_per_page']) )
 		$r['posts_per_page'] = $r['numberposts'];
 	if ( ! empty($r['category']) )
 		$r['cat'] = $r['category'];