I've been planning to mess with Google App pretty soon.
This dude ported the Google App Engine SDK to Amazon EC2.
No question that Amazon EC2 is the thing you'd actually use to do anything real, but while Google's price is free (for now), it might be fun to mess with.
Update: Re-reading this post, I realize it'd be much more productive to spend time experimenting with Amazon EC2 more, and really develop something valuable on it. It can do everything Google App Engine can do, and a lot more. It's an open garden for production apps, while Google's is much more closed toy playground. I probably shouldn't get distracted by Google's.