Well, this kind of sucks.  How does this even happen?

UPDATE!!! Commenter Salsa demanded a first-person account, so here goes:

well salsa, i was trackstanding at the stop light on valencia at 19th st, heading south. the light turned green, so i pushed down with my dominant (right) leg in order to accelerate adequately enough to address the small slope.

I feel some weird tension, hear a sharp SNAP, and then notice my control over the bike has diminished severely. i wobble to the right and manage to dismount without crashing. onlookers are stopping and gawking, many having assumed that cranks were unable to snap in such a manner.

the cop behind me passes me and yells out the window, “your pedal’s back there,” so i walk back into the intersection and collect it. then i take a picture.

UPDATE 2!!!  My bad!  The crankset is made up of Sugino components.  Sorry to besmirch your good name, Shimano!  This still sucks though . . .