Yeah, I just got to the bit of the video with the two separate Wave systems.
I'm still not sure what to make of it. I doubt it'll replace email, simply because some people somewhere are bound to refuse to use it because it comes from Google, the new Microsoft. For most people I think it'll just be another username/password to remember, and another page to keep open in a browser tab.
Plus there's a good chance that Google will at some point go "meh" and half-forget about it. A bit like Google Docs.
It all depends on what developers can make out of it, I suppose, and I ain't one of those.