Nature is supposed to put out 220 something billion tons of CO2 into the atmosphere a year. We put out something like 5 billion tons a year. If we disappeared off the face of the planet, the Earth's CO2 balance would barely be affected at all. Either it absorbs the CO2, or it builds up in the atmosphere. +/- 5 billion tons doesn't make a whit of difference - there is nothing special about human CO2.
http://answers.google.com/answers/threadview?id=258277
I got these numbers from this source. I'll need to check later to make sure they are reasonable numbers. For some reason, billions of tons seems a little too large compared to the Earth's atmospheric mass.