According to one source, it is perfectly fine for Microsoft to ban you if you have a fan mod on your Xbox 360. A user of '360patches' was reportedly told directly from MS reps that his fan modification on the Xbox 360 is in violation of the terms of agreement so therefore MS can legally ban users from live for that very reason. His 12v fan mod wouldn't too hard to detect as voltage metering is something which should be easily detectable though bios settings.
NEWS SOURCE: http://360patches.com/forums/index.php
NEWS SOURCE: http://360patches.com/forums/index.php