As long as I'm reflecting on our Google I/O experiences, I also want to point out what looks like a very useful library from Salesforce. The Force.com Web Services Connector is a toolkit designed to simplify calling WSDL-defined SOAP and REST services. The best part is that they have a version that works on Google App Engine for Java! (Make sure that you use wsc-gae-16_0.jar, not the regular version.)
I haven't had the chance to do a lot of development on GAE/J, but my colleagues have definitely had some headaches getting SOAP and REST calls working around the GAE/J whitelist. Maybe one of them can comment after we give this toolkit a whirl.