Index: xmlrpc.php
===================================================================
--- xmlrpc.php	(revision 9099)
+++ xmlrpc.php	(working copy)
@@ -2150,7 +2150,11 @@
 
 		if (is_array($catnames)) {
 			foreach ($catnames as $cat) {
-				$post_category[] = get_cat_ID($cat);
+				// get_cat_ID() uses prepare further down, so we need to
+				// prevent double escaping
+				$cat = stripslashes( $cat );
+
+				$post_category[] = get_cat_ID( $cat );
 			}
 		}