Spotlight menu shows results, but Finder window doesn't
It turns out that the problem was that I had the Finder window list set to sort by the Kind column. Spotlight apparently does not like that arrangement, and refuses to display results. I had to sort by Name, close the window, and reopen it. Now Spotlight works perfectly.

This problem was originally with Snow Leopard, and persisted on Lion 10.7.0, but seems to have been fixed at some point before Mountain Lion 10.8.4.