Well, the issue with cross platform multiplayer is control. Keyboard+Peripheral is always better than just a control pad. ALWAYS. That gives PC gamers an edge.
Example: Quake 3 was cross platform between the Dreamcast and the PC. PC gamers RIPPED APART the console players, because you can be more accurate and quick with a mouse+keyboard combo than the control pad.
Example 2: Universe at War was an RTS where there was cross platform play. Again, PC gamers could destroy console players because we have the keyboard mouse combo which allowed us to manipulate the intereface better than the console players. Hell, there was a 100 point achievement for the Xbox 360 players for defeating a PC gamer, that's how rare it was.
It's been tried, will never work.
BLOG: Keycaps are enroute. Yay.