Update to the update
There is an update to yesterday’s BC update. You’ll get the prompt to d/l the free update when you try to play an original Xbox game on your Xbox 360 (and you are connected to Xbox Live.) Don’t have Live? Xbox.com has a download that you can burn to a CD/DVD that contains the update.
This fixes the Halo 2 widescreen issue which cropped up last night after the update went out.