Update: This video is from 2011. Yikes. Sorry about that, folks.
A woman identified only as a "brave designer at the WCG office in San Francisco" (near the waterfront at 60 Francisco) saw either her, or a friend's bike getting stolen, in broad daylight, outside the windows of the office, and she wasn't about to let it happen. Watch as she, followed by a couple of coworkers, tackle the dude trying to escape with the bike. After being humiliated, he runs away. Yay for vigilante justice!
The action starts at the :24 mark. (As for how the suspect got the bike so easily, it looks like he used bolt cutters. You can see him put them in his bag at :32)