The final cylon at this point almost *has* to be Laura Roslin. Everything else would just make viewers go 'Meh'.

Plus I think Roslin is the only one it could be that wouldn't suddenly turn the w hole show into a contradictory mess.

Although I suppose it is possible they could introduce a major new character in the final season, and have the story arc being about them being the final cylon.

Maybe they'll get Original!Starbuck back to play him.