WordPress’s XML-RPC support looks pretty good. Heck, it supports a half dozen APIs and works well with ecto … except for tag support, which is my only complaint with it so far.
The Movable Type API supports a “keywords” field that I’m thinking can be hijacked as a “tags” field instead, but while ecto sends the goods — I can see them in the XML-RPC data that gets sent out, WordPress seems to ignore them upon receipt. So I’m looking around the WordPress plugin API docs for a solution, but all I can find is an undocumented mention of xmlrpc_methods in Skippy’s list of plugin hooks.