I havent changed anything, it must be a change withing mtgox.
Jup, I assume they did it because of the DDoS attacks - although I would guess this does not change a thing, really.
But they apparently really changed something, so you'll have to change your code.
That's exactly the problem with Mt. Gox - or let's say the biggest problem - they would not communicate if their life depended on it. Central change in the API? Why worry! Nobody needs to know...