Came across this interesting post just now.
Making XML-RPC calls from a Google App Engine application.
Looks like a useful technique. Google App Engine (GAE) doesn't support sockets, so anything that requires them, doesn't work - at least out of the box. XML-RPC is one such thing.
The blog author, Brian M. Clapper, shows how to use the urlfetch.fetch routine (of GAE), to make XML-RPC work on GAE, and then uses it in a blogging app that runs on Google App Engine.