Facebook's new-style messages service -- an abstraction of email, IM, and SMS -- is already working in the latest version of Facebook for iPhone... one you've had the feature rolled out to your account. Facebook isn't the first to try and redefine the ways we communicate online. Google has done it twice already, once successfully with the eccentric but reserved Gmail and again unsuccessfully with the wikichat that was Wave. Facebook is a social network however -- the social network -- so their take is decidedly different and whether or not it catches on remains to be seen.
Setup is simple on the web. You get an invitation, you accept, then you get your @facebook email address, verify your SMS number (if you want), and login to chat (also if you want). After that you're hit with the new interface. Once that's setup, the next time you launch Facebook on iPhone you're alerted that new-style messages are good to go (same way Places alerted you when it arrived) and you're good to go.
We'll be testing it out over the next week or so but in the meantime check out the screenshots after the break and if you have any questions or comments, you know where to leave them!