After Confirmed Dead, Jason raised the provocative possibility that Ben's man on the boat might be Michael. At the time, I cast my lot with Doc Jensen's equally provocative suggestion that the mole was actually Charlotte -- the bulletproof vest demonstration seemed like it had to be planned. But I had a brainstorm recently how Michael might be Ben's agent after all in a scenario that also explains Taller Ghost Walt.
Last we saw Michael and Walt, they were leaving the Island on a boat and bearing of 325, both provided by Ben. Thanks to Frank and Sayid, who departed on a bearing of 305, we know that which direction you leave the Island can affect when you return to the real world. My prediction, based on their experience and Faraday's rocket experiment, is that 325 took Michael and Walt several years into the past...
They found rescue just as Ben promised -- I have a feeling Widmore's freighter has been parked there for some time. Maybe Michael and Walt returned to the world and assumed new identities, but both soon realized that leaving had been a mistake. Or maybe they stayed on the freighter that whole time. However it happened, Ben re-established contact at some point and recruited Mike to be his mole.
Michael and possibly Walt are on the freighter. The former unlocked the sick bay door for Sayid and Des. And that really was Walt -- or rather, his ghostly astral projection -- that appeared to Locke in the ditch. Ghost Walt was taller because it really has been several years from his perspective since leaving the Island with Mike. Kudos to Jason for the call!