I like Car Talk's take on nitrogen in tires. Basically it's a scam.

Using nitrogen in the tires of some aircraft does make sense, though; FAR §25.733 (e) states: For an airplane with a maximum certificated takeoff weight of more than 75,000 pounds, tires mounted on braked wheels must be inflated with dry nitrogen or other gases shown to be inert so that the gas mixture in the tire does not contain oxygen in excess of 5 percent by volume, unless it can be shown that the tire liner material will not produce a volatile gas when heated or that means are provided to prevent tire temperatures from reaching unsafe levels.