Pleas read what FuFa wrote closely.....
Fufa's post above is talking about taking 12V from the DVD drive. This practice has been squashed for sometime as it is now known that taking anything from the DVD connection can and will cause damage and or banning from live.
I dont believe this warning applies to the 12V source that comes directly into the mother board from the Power Supply unit and here is why:
after having some brief discussions with software guys, we have come to the conclusion that it is possible for M$ to look at power consumption of you unit when it is connected to live. However none of us believe M$ is actively looking at power consumption for several reasons. First of all it would take too much to look at everyones console for this or even DVD firmware modifications, so to that I would think there are certain flags that set the banning process in motion. Most of the time I believe that has to do to poor quality ISO people are playing that sets off a flag for M$, once the flag is set then they have to confirm that the TOU has been violated. If they were having difficulty nailing you on the playing backups end of things they might look at power consumption in order to seal the deal. Another point is that why would M$ try to ban someone for extending the life of their product?
I have not heard of anyone being banned that didnt mod the firmware of the DVD drive. There are literally thousands of people with additional fans or 12V mods that have had no trouble at all with xbox live. So to that I say that you would be fine doing fan mods as it really is not any different then what the add on coolers do when they tap into the 12V power supply.
So yes microsoft most likely could tell if you using fan/power mods. No they probably wont ever know unless you provoke them into looking into your console. The possibility of a banning does exist, however the probability of it happening is close to zero.