The service, codenamed Project Titan, is supposedly a fully-featured email service that would give Facebook users a free @facebook.com email address. Obviously, having your email tied to your Facebook account would make managing your contacts a bit more streamlined, and I'm sure there will be an interesting refresh of Facebook's UI to integrate it.
But do you really want to do more of your online interacting through Facebook? With their shoddy history with privacy, I can't say I trust them all that much to handle my precious email. I think I'll stick with Gmail, thanks.