Personally, I believe that the paintballs themselves would have more of an environmental impact, if any at all, than the CO2 cartridges would. The amount of CO2 in those things is pretty small when compared to the CO2 produced by your car when you're on your way to the paintball meet. With all the movement and running around, your body will wind up exhaling more CO2 than those cartridges will give out.
So let's do a quick mathematical calculation of how much CO2 those cartridges will be given out. First we have to convert the psi into atmospheres. 1 psi contains about 0.068 atm, so the pressure of the cartridges is approximately 57.84 atm. The 20 ounce cartridges equals ~0.591 L. Using PV=nRT, we get about 1.39 moles of carbon dioxides which is about 61 grams of the gas.
To put this in perspective, a human being exhales about 408 L of CO2 per day. (www.cchem.berkeley.edu/chem1a/fall03/labsamplewrite.htm
). Using a pressure of 1 atm and a temperature equal to normal body temperature, the ideal gas law says that a human being exhales about 16.03 moles of CO2 which is equal to about 705 grams of CO2! So I would easily be able to conclude that the CO2 cartridges are no more harmful to the environment than a human being is just by breathing.