I think even if Petey is a playable character we won't find out until at the very least the day the game is released (unless there is a reliable leak). If he's in, he's probably an unlockable character, right? I mean, a new Mario bad guy, who is a boss in adventure mode? He screams unlockable (if playable).
And another thing, it seems to me that the makers of Smash Bros. try to make as much playable as possible. In SSBM there were few un-playble characters (Master Hand, Crazy Hand, Giga Bowser, Male and Female Wire Frames, Sandbag) and those were characters that it didn't really make sense to play as (the Hands had a life meter, not the normal percentage of all the other matches, Giga Bowser was just a big Boswer that was more a bonus to get to fight, Sandbag had no moves, and the Wire Frames were given no B moves, as far as I know). That said, Petey SHOULD have a full moveset, given his appearance in the newest video, and it would be silly, if not entirely a waste of programming, to not include him as a playable character.
Personally I do not consider Petey to be amongst the top 40 Nintendo and 3rd Party characters that should be in the game. However, if he's in it as a boss I would be very disappointed if they did not make him playable. Oh, and he'd better be able to fly in some capacity. Maybe not Peach style, but AT LEAST Kirby style!