I suppose that it is no surprise that San Francisco would take the lead in letting air travelers buy carbon credits to assuage their guilt for traveling in a jet.

http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2008/12/24/MNIR14PSQF.DTL&type=green

I wonder if anyone actually thinks this does anything to reduce the concentration of CO2 in the atmosphere. I suspect it does as much good as the indulgences sold by the Catholic Church during the Middle Ages.