We had a great tech talk last week Force.com for Google App Engine for Java that showed how to connect the two clouds.
There’s an interesting slide about 10 minutes into the talk which talks about the meat of the engine: Web Services Client (WSC) – a (freely available) Java-based high performance web service client which is part of the project.

I believe WSC can also be used outside of App Engine – and hope to get an article out on how to do that soon. Enjoy the webinar!