The news is that the Windows, Mac OS X and PlayStation 3 versions of this game will all use Steam for the online network and be able to play against each other online despite what platform you're on... for free.
While the XBox 360 version will be stuck with being played only over XBox Live and only against other XBox 360 players.

_{They even said the PS3 version will support using a Mouse and Keyboard and PlayStation Move as a controller to make things even when playing against PC and Mac users.}

(Source: Destructoid)