... and rebooting cleared it, so presumably a problem with Mac OS X 10.5.8's launchd, not with the game or with Steam.
I have now "verified integrity of game cache" in Steam, then deleted and redownloaded the game, and it fails in the same way.
Having just purchased Dungeons of Dredmor about 24 hours ago, this is only roughly the fourth time I've tried to launch the game (after having...
Separate names with a comma.