If it's a first person shooter then it would be very unfair. Mouse and Keyboard are a much more precise means of aiming that a little joystick on a controller. PC players would annihilate.
Counter-Strike: Global Offensive will feature cross-platform play between PC and PS3, but not Xbox 360, thanks to Micro$oft.
I cant understand why they are doing Multi platform anyway,Especially on a FPS game...like mentioned,Console players vs pc players?....Mouse accuracy vs console controller?...I'm sorry but i can already see the outcome.