It is easy to play with xmpp and the channel api to obtain a chat like...
http://www.dev-articles.com/article/Google-App-Engine-sending-messages-with-XMPP-393002
is an example, although I'm fighting a bit to understand the best way to manage the status of the users. Anyone with some experience about it?
I read the part "User Presence" in the documentation http://code.google.com/appengine/docs/java/xmpp/overview.html I still don't have a grasp of it...
Ok I think I know a bit more now, so this is how I manage the status of a xmpp account with google app appengine.