It's important to note that even though promotions and demotions are locked, ratings are still updated during the lock. This means if you perform significantly different than your current league, your league will change when Season 2 begins and you play your one placement match. If you go on a major winning streak during the lock, you may end up promoted, if you lose a lot more than usual during the lock, you may end up being demoted. Your games during the lock still count!
For more information on season lock and end dates: http://us.battle.net/sc2/en/forum/topic/2228179462