Seems more like a Google Wave Killer than a GMail killer if FB does this right.

http://techcrunch.com/2010/02/05/facebooks-project-titan-a-full-featured-webmail-product/

Yes it's maybe not real-time enough but who does real-time chats anymore? There's going to be increasing need for sharing of richer content that today's Email experience simply does not satisfy. I think Facebook realizes this as well as the merit of building good interop with email and other communication modes. Wave does a very poor job of this today.