Unfortunately, the Mac version remains $20, as Runic doesn't sell that version directly. The Mac version is only offered via Steam, and Runic informs Joystiq that Steam isn't participating in this particular sale. But don't lose heart, would-be Mac players, as Runic assures that a cross-platform sale will be held later this year (in case you missed the discounted Mac version the first time around). And don't forget: Torchlight will hopefully be out on consoles, too, before the year is out.
Update: Shacknews has discovered that the PC activation code for Torchlight can be registered on Steam, which unlocks the Mac version for play as well. Cheap Torchlight for everyone!
Update 2: Some users are reporting that the activation code is invalid on Steam, meaning neither the Mac or PC versions can be unlocked on the service. Shacknews reports that it used an older code obtained from Runic to confirm that it can be redeemed, however newly obtained codes may be invalid on Steam.