Oh, I doubt she's a Cylon. At this point, only minor characters could really be Cylons, or it'd spoil all their character development. And anyway, she's only tried to do what's best for the Colonials so far, if she was revealed as a Cylon they couldn't do a bunch of flashbacks reminding people "oh, remember when she did THIS?", etc. She hasn't done anything at all suspicious. The show has a lot of credibility because it shows flawed humans, people who make mistakes or do selfish things because they're misguided. Turning around and saying "ha! they were evil Cylons all along" would ruin that whole dynamic. I think Roslin will die(and soon, if they're really going to find Earth by the end of the season), but she won't be a Cylon.