I have no experience with any of these and they are all alpha/beta:
That said, I think people are overreacting and taking pessimistic conclusions too soon about App Engine.
I've looked at cloudfoundry. The open source version you can install locally was meh, but I'd hope what they've got running on their own cloud service worked a lot better.
I just run a VPS myself, because I do much more than java software and like to play around with the entire stack.
Fair enough. "Cloud" is a vague and often misused term, but there is a generally accepted definition of cloud computing (pdf).
Cloud Foundry is an open source Platform-as-a-Service software project. There's a pretty good description in the FAQ.