NPR had a bit this morning on carbon offsets. IMHO these are a pretty poor way to address the problem of carbon output. Most of these credits go towards things like wind and solar power and often claim to help you be "carbon neutral". These may reduce growth of carbon output, but they don't actually help remove any of the carbon you spewed in the activity you took part in (and for which you are buying credits). Its mainly a guilt alleviator, and really just serves to enable people to not improve or better their behavior or lifestyle.

Think of it this way, would you support a murder offset credit, that let you kill someone, but then donate money to help starving children somewhere (who otherwise would die). In the end you may be "life neutral", but you still did bad thing, and you can't right it by paying someone to make you feel better. Extreme comparison for sure, but I think it is apt.