Mr. Green is the Sierra Club's answer guy. He has a column about all things green that he updates weekly. In honor of Bike to Work Day, he's put out this discussion of the carbon emissions of biking vs. driving. He puts to rest the myth that fueling a bike (by feeding a person food that has been grown with fossil fuel inputs and traveled to them) is worse for the environment than fueling a car.
Read the discussion in Hey Mr Green, is driving better than cycling?. A hat tip to Carolyn in San Francisco for this tidbit!