Mike Berners-Lee posts on the Guardian his findings.  As it turns out riding a bike is about 1/10th the carbon footprint driving a car when taking into consideration the full life cycle of both transporation choices.

Mike presents a nice discussion on the points to consider when making this comparison, such as, differences in quality of life.  The full blog post can be found here on The Guardian website:http://www.guardian.co.uk/environment/2010/jun/08/carbon-footprint-cycling