Every MS product has that holographic silver seal around the edge fo the casing. Look at your 360, you'll find it. If that's broken, you've voided the warranty.
Talking about it here on these forums is a TOU against the forums, that's why I can't tell you how to do it. But as far as actually doing it? Is that a TOU against Microsoft and/or XBox Live? It's a bit of a grey area, since there is a way to prove it, but it's currently unknown whether or not they check for it. Tons of people have done it with zero problems, but that doesn't mean they won't have issues with the next banwave/fall update. --- PS3-It only does everything. Except work online.