Back on topic, I haven't been able to discern if the multiplayer will allow Mac users to play against PCs. Back in the day, DN3D Mac was unable to connect with the PC version, so the MP opportunities were already pretty thin. If that hasn't been addressed in this game, I can't imagine that anyone would even bother, even if the MP was better than it seems to be anyway.
From what I've read, Dukematch isn't all that great to begin with...