I haven't really seen much of it, but I'm intrigued by the idea. That said though, I fear it might not exactly be the next gmail. It reminds me in many ways of the early Microsoft "Active Desktop" push, along with IE's "push" websites. Nothing caught on there until years later when RSS feeds finally took off. Wave seems like it might be useful, but only in the same way AIM is... that is to say, it only works if everyone is using it.
Anyway, thanks to a friend, it appears I now have some invites to hand out. So if you're raring to get your paws on the newest google noun/verb, lemme know, and I'll toss a few out here. If, on the other hand, all of this was gibberish to you, then don't panic... you'll likely never miss it. :-)