If you want to sign up and start playing with Google App Engine and already have a Google Apps account (like a Gmail box using your domain) you may be experiencing redirect loops on your browser. I saw it with Firefox and Opera on Ubuntu and it seems to happen on Safari for Mac too.

The url to access Google App Engine is http://appengine.google.com but when using Google Apps you should use http://appengine.google.com/a/your-domain.

Presto!