back in the early days of games for windows live MS tried a few cross platform experiments, Lost Planet, BlazBlue: Calamity Trigger, Uno and Shadowrun being the only games I know of. Apparently the PC players dominated so much they abandoned the scheme.
http://www.eurogamer.net/articles/ms-killed-pc-xbox-cross-platform-play

They definately don't encourage it and the fact that the PC version has to be GFWL limits the feild, I don't think they forbid it tho. BlazBlue was released 2010 on PC making it the most recent example.

Also both versions of the game have to be the same, and considering the limitations on patches and such on GFWL that hamstrings, the PC version. 

Obvioulsy the pc would destroy the controller weilding enemies so they should have balanced it with some sort of auto-aim or if, I'm assuming it was 4v4, make it 2 pc and 2 console per team at the very least.

Keyboard and mouse wipe the floor with controller precision. Doing the cross play really puts the pc players at an advantage! I think the next counter strike, global offensive or something is said to feature cross play between consoles and pc. Not sure if they're still going ahead with it though

yeah i am happy we don't have that for more games. it really destroys the fairness of the game. sure you can give console player auto aim and pc players not but it will never be balanced. it's bad enough that some people play with mouse and keyboard on console against controller players. i don't get why they do it, it has to be freaking boring knowing you have a huge advantage against most. ok i do the opposite, i play with controller against others on pc but yeah, it shouldn't be the case for console games.

maybe we should get this for co-op play in some games like borderlands or so but please never in games against each other.

CS GO was suppose to be cross play with PS3, like Portal 2 was, but it got taken out on CS GO as it was impossible to balance.

Ya MS tried it. Never caught on though.

pshhht, i doubt pc players got much of an advantage in Uno.

The first game I remember playing that could do PC/Console was Quake 3 on Dreamcast/PC. PC player could download a patch to play with Dreamcast players. It worked, but PC player had a huge advantage with the precision of a mouse and keyboard. The same advantage keeps it from being done today. To make it "fair" PC gamers would have to be handicapped or console players would need minor auto target. But that wouldn't work either because nobody would like the idea of altered play.

I don't think it would be worth it to developers to spend the time and money developing crossplatform play that only a handfull of people would use.