the 360, PS2/PS3 and PC versions all play together. As a matter of fact, there's one of each of those in our house playing together
My hubby is the 360 user and he loves it. The only bad thing about it (in my opinion) is one time he wanted to use my account and couldn't because his gamertag is linked to his Play Online account. However, before we got him the Elite and he was dealing with RCoD 360 issues often, he could fire up the old PS2 and play on. He's played on all three (four if you include PS3) possible connections and prefers the 360.