I never thought I'd have to say this, and it makes me feel kinda dirty after reading the above post, but Crazy! is talking the most sense in this thread. It's blatantly the opening movie for the game, and it's pretty clear that at least some of it is pre-rendered stuff.
As for someone else being the boss... Err... Bowser was the boss of Smash Bros Melee? Master Hand was only the boss of Classic mode, which really felt more like an extra to me. Adventure was the real 'story' mode in Melee while Classic was sort of just a homage to the first one. I'm betting (Hoping) that Brawl will have Story, Adventure and Classic modes (Probably All Star too) so Master Hand will still be there, just not as the boss of the main story mode. Also, if you remember Master Hand manipulated dolls in the original, not statues, so he shouldn't be the main boss of either Melee or Brawl ;3
I'm hoping that if they have Adventure and Classic in Brawl they do a similar thing to both as they did to Classic in Melee. As in, in the original Smash Bros the 'Classic' was a set of specific fights, with stuff like target test in between. Then in Melee they kept Target test and Race to the Finish, but the actual fights were randomised, the metal character could be anyone rather than just Mario for example. I'm hoping that in Brawl they'll maybe randomise the order that you do the Adventure levels in or something to make it a bit more interesting, or make it so that the final boss is a slightly larget version of any character, or maybe your own character's rival, rather than just Bowser every time.