Flickr user JM3 reports a heck of a time finding a Muni pass -- despite going to a Muni station to obtain one, he had to get on the train and ride a few stops before reaching an actual vendor. "You almost have to HAVE a muni pass in order to GET a muni pass," he says, which is still only a partial list of requirements: you also have to have the secret unauthorized Muni-map hack to find a vendor nearby.
On certain days, you can also buy monthly passes online -- but you can't buy weekly passes, or use Commuter Checks at all. That functionality is "coming soon," like oh so many promised improvements at Muni. August is the latest estimate we've heard for online Commuter Check usage.