Michael is in the coffin.

When you piece it all together it all makes sense.

• Michael wants to die, and the last episode was all about how the island wouldn’t allow him to die – yet.  This is all leading up to the flash forward of last season when Jack saw the obituary in the paper.  My money is that the name listed on the obituary is his fake name, Kevin Johnson, since Michael can’t reveal that he’s one of the survivors of Oceanic 815 and Ben has provided false identification (passport) that proves him to be Kevin Johnson.
• We know Kate is really upset at the person in the coffin.  Michael not only killed Ana Lucia and Libby but sold out Kate, Jack, Sawyer.
• We know the person in the coffin has to be someone that made it off the island and not one of the Oceanic 6.   If it was an Oceanic 6 member this death would have way too much coverage.  We know from the last episode that Walt won’t talk to him and his own mother is really upset at him.  All of this explains the lack of people at the viewing.
• I believe the actor that plays Michael is only listed on the show as a guest star and not a regular cast member.