According to Destructoid, Army of Two's online co-op mode is limited by disc region. What exactly does that mean? That means if you've got a buddy with the game in England, and you are located in the US of A...well you are SO of L. The lone clue to this fubar situation came from the PAL version game's manual.
"NOTE: The online game modes in Army of Two are not cross region compatible and is only supported between PAL discs."
While not a big deal to most people, it's still suck-full situation for those of you who have international friends that you game with. Thanks to Xbox Live I've personally made some aquaintances located in the European and Asian markets - looks like I won't get to do a fist pound after a massacre with them.
Source: Evil Avatar
Update: Joystiq got the explanation from EA regarding the situation:
According to Schneider, said super slow connections are what prompted the region blocking. Using a deterministic (peer-to-peer) network model enables the game's synchronized gameplay animation, but requires parity on both systems. If a slow connection is thrown into the mix, "the person with the worst connection brings down the whole group to his/her worst connection level," explained Schneider. "While not generally a problem for COOP (since most people play coop with friends) when playing in Versus mode 3 people can be highly adversely affected by one player with a bad connection. We use the region lock to minimize the likelihood of this occurring."