I had a chance to speak with Torchlight II developer Runic Games about some more details regarding the Mac version of the game. There will be a Mac version, but disappointingly it will not be released at the same time as the PC version. In fact, the date will not be determined until after the PC version is released!
Regarding multiplayer, they can’t commit to PC/Mac multiplayer co-op play yet…it’s still too early to tell. But they are working on it with the intention of making it happen. So, cross your fingers and light candles on your Mac Pros, peeples!