The main argument for nitrogen as a tire inflation medium is that the molecules are a little larger and are less likely to penetrate the rubber and valve causing the pressure to decrease. Air is about 78% Nitrogen in nature, with most of the rest being 20% Oxygen and about 1% Argon. If you check your tires regularly and reinflate to compensate, the use of pure Nitrogen has no effect other than the added cost of the original fill of N2. When you stop to think about it, what is leaking out naturally is the Oxygen and Argon, meaning your "air" is coming closer to pure Nitrogen anyway.

My suggestion is not to bother with anything other than the air we breathe. Check your tires' pressure every month or so, and while you are checking, look for tire wear and the stray nail that may be in your tread, increasing your rate of pressure loss.

And if you are really concerned with impressing your enviro-neighbors, you can buy green valve caps on the Web and screw them on yourself! [g]