Yeah, I'm pretty sure it's been confirmed that there's no cross-platform play...but even if it hasn't been confirmed, it won't happen, as Microsoft's Live service is direct competition to Steam.
I do think this game could be played reasonably well with a gamepad though...but I'm sure a mouse and keyboard would make things much easier. Imagine trying to precisely dangle your Smoker's 50 foot tongue and grabbing a Survivor with a gamepad...I can't.
LEFT4DEAD411.COM
Bet with disolitude: Left4Dead will have a higher Metacritic rating than Project Origin, 3 months after the second game's release. (hasn't been 3 months but it looks like I won :-p )