After the recent fall dash board update, many users are reporting non-functional 360's. This includes units that are overheating, locking up, graphical glitches (checker board patterns) and even system death after prolonged use. Microsoft is refusing to admit the update is the problem and are asking $140 for repairs, even though hundreds of people are having the issue after the update. See the link below for more details on this situation.
Xbox.com Forums
Xbox.com Forums