There's a lot of speculation that Michael is Ben's inside man on the boat. I think this is based on nothing more than the fact that he headed off into the sunset and we have no idea what happened to him. In my opinion, it would make little sense at all for this to be true, but it's not impossible..
For one thing, if the crew of the boat fished him out of the ocean he wouldn't be put in any kind of position of trust by the crew. On the other hand, he's on the manifest and probably has a good story as to why The Others released him. Also, it's not as if Ben has specifically stated that he has a Patchy-quality spy on board. The information about C.S. Lewis is pretty impressive, but it's all stuff you'd find on a resumé. Within 5 minutes of an honest conversation with Michael, it's pretty likely that he'd reveal a bitterness regarding Ben, prompting the crew to share this part of the mission with him.
I don't see how or why Michael would get this info back to Ben, though. These two questions are the source of my doubt about him being the inside man.
My biggest reason for thinking it might be true, though, is that Harold Perrineau's name is in the credits. Since he wasn't shown, I can only guess that he may have been one of the voices on the radio or he was a whisperer or something.