Actually, the system probably detects flats by comparing rotation of the wheels detected by the ABS.. not by sensing pressure. The dealer might be able to reset it.. otherwise, you might have to take a *long* drive for the system to realize it's ok (I think my Scion manual said it took 40 minutes of travel before its system got a good baseline -- but.. at least it had a reset button!)

My roommate's Corolla has had the flat warning light since three weeks after he bought it (the reset button didn't work for him), and the dealer has looked at it twice, So.. GOOD LUCK!

-K

Edit: Just looked it up for myself.. it's really weird that the manual lists such a specific low pressure reading (28psi) that would activate the system, when the system has no sensors to detect the actual pressure! --Very misleading-- The original Hummer was the only vehicle, to my knowledge, that could actually detect (I think it could also adjust) the pressure in the tires. That would be killer.