I wouldn't count on PC/360 online play though...how does Live Anywhere work?
Left 4 Dead is probably going to be distributed to Steam...and I can't see Microsoft working through Steam to allow PC/360 online play. I'd just get it for PC if I were you.
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 )