While Flatland2d and SVOBoy are conducting their experiments on overinflating the tires, I would like to mention a very cool product that I'm considering buying in the future.
According to the EPA, each 2 PSI your tire is underinflated will cause you a 1% drop in fuel economy. WHile this isn't anything drastic, imagine what your mileage would be like if your tires were 10PSI low (5% - on a 40mpg vehicle, that's 2mpg - or 20 miles on a 10 gallon tank - you're wasting).
The other day I filled up my fiance's tires and one of them was at 10psi. Her tires are rated at 35psi. Neglecting tire pressure is probably the most common gas mileage offender.
In my old Sentra, I never checked the tire pressure. I also never rotated the tires, balanced them, etc. I thought, "I have a new car, why would I need to check this stuff?" Well, I also went through 3 sets of tires in 3 years. In retrospect the main culprit was under-inflated tires, which caused the sidewalls to crack and caused extreme wear on my tread.
In other words, tire pressure is super important, but you already know that.
On the commercially available Hummers (H1) they have an onboard tire pressure monitor with a small air compressor that will refill your tires when they are low. The H2 and H3 have the monitor, but no air compressor.
After a little searching, it turns out you can actually buy these tire pressure monitor systems yourself. For $300 installed (probably much less if you install it yourself) you can have little radio transmitters on each tire stem that transmits the tire pressure to a small receiver in your cabin. When they are low, an alarm is sounded.
Some products and information are here:
Looks like you can get them on ebay for around $70 shipped.
click here to see
Given, it is much easier to just check your tire pressure every few weeks, and much cheaper. I think though that adding these little gadgets is kind of fun, and make your car much more like knight rider :P