9to5Mac (via Gizmodo) shows Push Notification already working in the latest version of Tap Rap Revenge [Free - iTunes link] when run on the iPhone 3.0 GM developer seed. Gizmodo, of course, raises the pertinent question:

It certainly isn't the most creative use of background notifications—the messages only pop up when you're challenged by another player—and I'm already wondering how, when all kinds of apps have this capability, the iPhone's exceedingly simple notification system will scale.

Since Text Alerts don't tack, a new one obliterates the one before it. What happens when an urgent IM is replaced by "Hey, dooooooode, can haz rock out?!" Sure, badges remain, but then you have to go looking for them...