Nitrogen is inert and doesn't affect rubber or the steel rim.
 Carbon dioxide is highly reactive and affects both the rubber by causing swelling in the rubber. It also would cause corrosion in iron based rims (particularly Carbon- Steel). 

In the case of automotive tires using CO2 would probably cause catastrophic failure. My experience with Nitrogen is that I have noticed it to be much more quieter while driving and less pressure variations when the temperature varies from season to season. 

http://encyclopedia.airliquide.com/encyclopedia.asp?LanguageID=11&GasID=26 

Check under Material Compatibility under the heading of Elastomers- Quite illuminating.....