When it comes to mods and warranties in the U.S., the dealer/manufacturer has to prove that the modification is the cause of the failure in order to deny warranty work.
Read the following:
Magnuson–Moss Warranty Act:
http://en.wikipedia.org/wiki/Magnuso...s_Warranty_Act
Keeping Your Mod's Warranty Intact:
http://www.dummies.com/how-to/conten...ty-intact.html