I'm not convinced it's Michael either. It just seems way too obvious. And as many pointed out, why would the freighter people pick him up and trust him to help them? Wouldn't they figure someone escaping from the island might be working for Ben? And a better question - why would Michael be working for Ben once he was off the island? I can see why he'd work for Ben in order to get Walt back and get rescued, but once he's gone, why would he still be working for Ben unless Ben threatened that he could still "get to him" if he didn't do what he said. Doesn't make sense from either angle.
I guess it's possible Michael could be stowing away aboard the boat somehow and acting as Ben's spy without the boat crew knowing it, but again - why would Michael still be working for Ben? How would they be communicating? Walt?
I'm more inclined to believe that it's one of Ben's guys that wasn't on the island (like the one that Widmore captured). So... who's someone that we expected to never see again (that would have some impact on Locke so that he had to "sit down" when Ben told him) that might be working for Ben? Could it be Locke's Dad? Maybe the man Sawyer killed that "appeared in the box" wasn't actually Locke's Dad, or at least not Locke's dad from this reality/timeline. Maybe that man was just another one of the island manifestations like Eko's brother, Jack's Dad, Kate's horse, etc.
Speaking of Christian, could it be Christian? The actor already "appeared" in the cabin Hurley saw in the premiere -but how could the writers explain his presence on the boat? That just seems too convoluted to me.
Some of the other suggestions I've heard (Locke's old boss Randy, Charlie, etc.) are also interesting... but at this point, I basically think it could be anyone - dead or alive. Obviously I'm really going out on a limb here.
Any other crazy suggestions you can think of? How about Ben recruits Boone from an alternate universe and gets him on the boat? Boone did mean something to Locke...
