It would be swell to see an XML-RPC API (any XML-RPC API!) implemented in WordPress that supports categories. The MoveableType API is listed as supported, yet the very standards-conscious NetNewsWire news reader and blog posting software throws an error when attempt to retrieve categories via the MT API from my WordPress blog. The Blogger and MetaWeblog APIs work fine for editing recent posts and posting new ones without categories, but workarounds like those described in a previous thread on using NetNewsWire and WordPress are clumsy.
I haven't delved into the xmlrpc.php yet, but I imagine that category support would not to be too difficult an addition, if it isn't already supported for other blogging clients like Zempt.