He's basically some dude who tried (and failed) to run as a district supervisor in SF, and has since made it his cause to tangle up any bike-related improvements to the city's infrastructure. Basically, through his efforts, city transportation agency's plans to pave a bunch of new bike lanes have been delayed till at least the middle of '09.
I might be mistaken on the specifics, but that's the gist of it. I'm sure someone here could speak on it more clearly.