Michael Levi of the Council on Foreign Relations weighs in on the role of gas in reducing US carbon emissions. http://blogs.cfr.org/levi/2012/09/25/why-have-u-s-carbon-dioxide-emissions-plummeted/comment-page-1/#comment-166725.
Michael's analysis discusses at length my September 17, 2012 posting on the same topic. See johnhanger.blogspot.com/2012/09/natural-gas-is-responsible-for-about-77.html. I have also commented twice on Michael's analysis at his blog.
The role of gas in reducing carbon emissions differs in different time periods. It was significant in 2011 but even bigger in 2012, for a number of reasons, including gas is displacing even more coal generation in 2012 than in 2011, and renewable electric production is down in 2012 compared to 2011.
The plummeting of carbon emissions in the US is real and not a result of magic. Gas is at the heart of the good news, and it is sad to see some deny that fact, as others deny the reality of climate change.