In response to a comment by Stephanie, here is a recent article at the NYT regarding the recent decrease in C02 emissions in the US. It seems that multiple factors are behind the decline: relatively mild temperatures last winter, the recession reducing the demand for heating fuel and the increased use of natural gas by energy plants (due to its recent price decrease). In other words, the demand for energy sources that produce a lot of C02 decreased for 3 reasons.
The price of natural gas has been rising more recently, but economic growth remains sluggish. It should be interesting to see if this trend continues. Here is a look at emissions reductions in the US and EU from the Energy Collective, as well as some insight into potential causes for the declines.
Regarding the costs and benefits of a cap and trade program in the US, I found this short piece from the Brookings Institute, this short article at the NYT, and this summary of research from MIT (examining the distributional consequences).