They will be starting that. There's a few games coming out, including UT3, that will allow cross platform play. I personally don't like it though since everyone knows the Keyboard and Mouse is superior to the Console Control in FPS. It wouldn't be very fair overall. I love playing FPS on my console, but only against other console players. I don't want to get owned by some kid on his PC :)
If they released for the 360 that OCZ control that hooks around your head and monitors eye movement, brain waves and muscle twitches to aim and fire at your enemies, then i'd play PC gamers all day :) That thing owns.
http://youtube.com/watch?v=O7EnLXGfA18
http://youtube.com/watch?v=_Mg84z0bcuE
http://youtube.com/watch?v=TDiFX89VfMo