Remote tire pressure monitoring system

http://www.satetpms.com/productsPer.asp?id=16&className=TPMS%20for%20Car

The system monitors tire pressure wirelessly, and a unit attaches to the tire-stem. The unit needs to be recharged manually. The solution it would seem they want, is to use the rotational energy from the car moving to recharge the unit.
Even better than that, the electronics on the tire-stem would be passive, and are powered via a transmitter in the wheel well.

I'll bet, however, that its a cost thing, as this unit only costs $129, which seems relatively reasonable.