[photon]

Depends on how the Xbox is being modded. If it's a physical mod of course not, but if it's a software modification then sure they should fix it. Should they be able to refuse a warranty because I played a specific game?

This might lead to an interesting court battle, Apple can say they will void your warranty in their EULA, but just because it's in the EULA doesn't mean it'll stand up in court. It doesn't seem reasonable that Apple would withhold a warranty on the physical integrity of a device based on it running different software, or at least the onus should be on Apple to prove that the software caused the defect. EULA's aren't a carte blanche to say anything they want, it still has to fit within the law.

Take a car warranty, VW can't void my warranty because I put in an aftermarket stereo. If they suspect the stereo blew something out they have to have reasonable grounds to refuse the warranty.