CLEAN UP! AISLE 2!!!!
Jeebus Kurt, spelling, punctuation and capitalization aside, at least please try to use all the words....
Anyway, what I think Kurt's trying to say is that it would depend on the dealership where you are having the truck serviced. Now, the warranty only comes into play when affected components could fail due to the headers.
Magnuson/Moss Act states that the dealership must prove the unapproved part caused the failure. So, if your bumper falls off, they can't void your warranty due to the headers.
That said, it's commonly known that MANY (if not all) states have legislation in place to safeguard against any tampering of emission control or pollution deterrant systems on vehicles. So, if you mess with the cats, which is inevitable when playing with headers, you run the risk of voiding your engine warranty.
The JBA cat4ward units require you to actually CUT your stock cats off the exhaust manifolds and REWELD them onto the tail of the headers.... That just seems to increase the possibility of problems if you don't do it perfectly.
My take is that since the cats are so close to the exhaust manifolds on this truck, and the engine has been known to suck in parts of the cats when they implode, thus destroying the engine, then it makes no sense to me to add headers knowing that there's a 100% chance that the dealer will void your replacement warranty with them installed, should anything happen to your engine. Now, if you move the cats further away to minimize this possibility, or otherwise safeguard against it, then it might be worth the risk.
Any way you look at it, if you mess with your headers and cats, and have an engine problem, M/M Act or not, you're going to be in for a fight with most Nissan service managers, just because it's money they don't want to spend if they can hang the bill on you.