Using similar techniques as Gmail and Blogger, I suppose.

I think I heard someone say whitelisting, but I lost the reference.

Well, if you're going to try to reinvent email you've got a better chance at making whitelists useful.

Even if your inbox (or whatever the wave equivalent is) could divide incoming waves based on whether they were from whitelisted contacts or from unknown contacts, it gets a lot more useful if people whitelist each other by default.