My dealership (local GMC place) said the tire pressure sensing system can't be turned off -- probably a federal requirement or whatever. Since I put other-than-stock wheels on my truck for the summer -- I fix the annoying little yellow light on the dash with a piece of electrical tape.
By the way, Bob -- I don't think you can retrofit the sensors into wheels other than those built for 'em -- at least not according to two or three different vendors. It's something about the shape of the hole and something else, I think, I remember, sorta.