Using AppEngine XMPP for Client Notifications
In that situation, I would perform ajax calls every 5 minutes in example to check it.
It's easy to implement and the data exchanged can be reduced to the max (taking advantage of "fast query/response" bonifications of google-app).
Regards.
I've been looking for a way to tell clients about expired objects and AppEngine's XMPP implementation seems really interesting because it's scalable, should be reliable and can contain up to 100kb of data.

But as I understand it, before a client can listen to messages, he should have a gmail account. That's very impractical. 
Is there maybe a way to make temporary readonly XMPP accounts to use with this?