I use metaWeblogAPI(python) to post on my wordpress.com blog. WordPress(<2.7) returns the name of the category for the "description" key. This is in conformance with the MetaWeblog API as specified in http://www.xmlrpc.com/metaWeblogApi
For 2.7RC*, the categoryName key returns the name of the category. I could see that the change was done with changeset(http://trac.wordpress.org/ticket/8494).
Wouldn't this change break the existing metaWeblog clients?