Using AppEngine XMPP for Client Notifications
No this isn't true: you can have the AppEngine robot as contact over any Jabber/XMPP based networks.

Unless you are talking about the need for a GMAIL account to create an AppEngine robot... in which case YES you need to have a Google account.
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?