Recently, LOST's producers fielded fan questions through Entertainment Weekly,  one of those questions was what would become of the characters of Michael and Walt, last seen on LOST boating off to what Ben assured them was their "rescue".  Cuse and Lindelof mentioned that their story would be revisited in Season Four.  That may have changed in the meantime, according to the Hollywood Reporter, leaving yet another LOST storyline flapping in the breeze.
Seems LOST's Perrineau was deep in negotiations to reprise his role as Michael in season four of LOST, but in the words of Hollywood Reporter "...an agreement could not be reached."  Perrineau has since busied himself with film roles such as 28 Weeks Later (the sequel to the horror film 28 Days Later) and more recently has signed on to a potentially long term contract with CBS which kicks off with the pilot for 'Demons', a primetime horror offering which has Perrineau cast as one half of a team of monster hunting priests.

At the time Perrineau and co-star Malcom David Kelly (Walt) departed the show, there was a flurry of rumors concerning the circumstances.  For Malcom David Kelly, it was the problem of integrating his growth with the one-episode-equals-one-day format of LOST.  Rumors circulated amongst bloggers that Perrineau had been upset with the bonuses that were handed out to select cast members, a situation that has never been widely rejected by representatives from either side of the LOST camp.

It is possible, of course, with the way Michael and Walt's story was left, to never address the story again and just assume what happened was precisely what Ben promised:  rescue from LOST island.  On the other hand, the increasingly agitated fan-base of LOST will likely react negatively to news that yet another story-line may pass without closure.