Versions
Just to clarify a bit, because the current posts are a bit confusing (at least to me). It is possible to play cross platform trough Steam, with games like Borderlands, but only when both are on the same version. Aspyr is a bit behind when it comes to update cycles, so if a patch is released on Windows, you can't play Mac-Windows, until Aspyr updates to the same version number, even if the patch only solves Windows issues. (When the game tries to make a multiplayer session, it checks if the string for the version number is the same, and if it is not, the connection will not be made).