Does it actually SAY that in your manual?
If so, I've never seen it. Besides, according to the Magnuson-Moss act, the manufacturer MUST honor warranty in the form of the vehicle delivered. The only thing that voids a warranty, per se, is stuff having to do with title.
They (the manufacturer) have the electronic capability to limit damage to your engine, therefore, by implicitly giving you a car with a fuel cutoff at 7000-7200, they are giving you a car that is warranted as delivered - even if you run the car around at redline all the time...
Modifications done to the car will not void your warranty, either. Depending on the failure, certain work MAY be denied if it can be reasonably proved that your modifications caused the failure - otherwise, the MANUFACTURER MUST HONOR THE WARRANTY.
I've heard this several times, though, and I wonder if it is just urban legend. If you have read this in your manual, though, angelqute1, could you scan and attach it? I'd really like to see it.