I believe AT&T will unlock the iPhone after contract is done. Whether or not you could slip a Digicel sim card into it then and have it work, I don't know, but I think so.
Cell service in Haiti is pretty good. You'll want to go with Digicel. They have prepaid and contract services available. You'll likely have to arrange a contract, if you elect to do that, at their main office in Port au Prince (if it's still standing). I spend a lot of time in Haiti, and my approach has always been to just buy a $35 Nokia prepaid. Calling the US over that phone is dirt cheap, and text messages back and forth are free. I understand you would prefer to use your iPhone, so I'd check into your options for getting it unlocked.
Where are you going to be in Haiti?