just a few more thoughts:
if a tire is inflated with cold gas, and then warmed up, the gas pressure will increase (like leaving your bike in the hot car on a sunny day.... the tire will explode eventually). If you fill your tire with He (I hope no one was planning on this anyway....might also get a little costly) the He will diffuse through the tire walls really really fast... and this really doesnt help out on the road.
Petra