At ProcessOne, we have developed an experimental notification Google Wave XMPP gateway.
I wrote it because, in the current state of Wave usage, my contacts always had to write me an XMPP message or send me an email to explain that I have content available.
I felt this was for me a large deterrent for using it and quite paradoxical in a real-time web world.
To solve this problem and benefit from the largely used XMPP protocol for notification, I wrote a Google Wave gateway. For now, it is quite raw because it relies on GWT client protocol and lacks any mechanism from Google to do delegated authentication. However, we are going to implement a more standard client protocol and delegated authentication as Google makes those features available.
Anyway, here is a small demo of how to use it to give you an idea of the user experience.
It is deployed on talkr.IM server, so you can give it a try.