The finale was pretty good, although I must admit I was slightly dissapointed by the very end of it.
I think what you mean by Matthew Fox playing another character is when he was at the running track/stadium just before Desmond talks to Penelope. That is in fact Jack going for a run after the end of a hard day at the hospital: you see him in his scrubs, and earlier in the show we see Jack talk with Desmond that same night.
EDIT: I just figured out you probably meant one of the guys in the Arctic/Antarctic, who looks a lot like Matthex Fox. Apparently it isn't him though.
