Just throwing this out there, I know some of you may have already done this but if you go to imdb.com there is a list of the cast and the characters they play including the number of episodes they've been in and in what years they've acted for those characters. For example during the 100th episode Jack would've read
Matthew Fow Jack Shephard 100 episodes 2004-2009
Imdb has been updated though for the first few episodes of season 6 and there are some interesting characters listed under 2004-2010, some which should read 2004-2007, lets just say there are some dead dudes coming back. Now I dont want to be a spoiler so check it out for yourself here: http://www.imdb.com/title/tt0411008/. Now none of the returning people surprised me except for the explosive return of one particular doctor. What did surprise me was some of the dead ones that are'nt listed as coming back that I was under the impression were supposed to.