It's common sense that if you have a modded Xbox 360 you don't play games online before their release or you're just asking to get banned. It seems as if a bunch of people playing burnt copies of Gears of War 2 and Call of Duty: World at War either before the official launches or just after, are now getting banned from Xbox Live. And apparently even the latest firmware versions may not be safe. So if you fall into this category I would suggest staying away from Xbox Live for a little bit.
source: Xbox-Scene.com
fuxx microsoft!