The 05-06 X has sensors in the wheels that detect pressure. They are mounted to the back side of the valve stem.

The Hummer also has sensors, and yes atleast the H1 could inflate and deflate the tires from inside the truck. I don't know if the H2 and H3 have the same system.

Do a search of this forum for "tire light" there are atleast two threads that have a lot of good information.