Dan Frommer:

We also anticipate — but don’t know — that Apple may build some sort of Facebook messaging features into the iPhone. Perhaps you’ll be able to send Facebook messages to contacts just as easily as text messages. This wouldn’t cost any money — it wouldn’t use up your text message allotment — and could be just as instantaneous. But again, this part is pure speculation.

I could see Facebook and Apple integrating some contact syncing, much like they do with MobileMe. I however do not see these messaging features coming to the phone, though I may be made to look a fool shortly.