You didn't have to have a finished game to play in the tournaments, though. All you needed was to be far enough in the game to trade Pokemon to your cartridge and you were set.
Though with HGSS most people already had lots of trained Pokemon on Platinum\Diamond\Pearl carts that they could easily transfer over. With B\W, it'll probably be new enough that most players won't have built up a collection of 5th gen Pokemon, though. I imagine most people are going to be transferring Pokemon over from their 4th gen carts, which is probably going to require you complete your game first.
I have a feeling U.S. players will be scrambling to get teams together for this year. But we at least have the opportunity to see what Japanese VGC teams will be like before the tournaments get here.