Portal 2 to be cross platform for PS3, PC & Mac

Purchasers of Portal 2 on the PS3 are in for a treat. Valve have announced that players will be able to chat and play multiplayer between the PC, Mac & PS3 platforms. In addition to this, if you get the PS3 version then it comes included with a voucher redeemable for the Steam version of the game providing that you link your PSN & Steam account.

Buying a console game and getting the PC/Mac version is a welcomed feature but why has Xbox been left in the dark?