When we first heard Army of Two's multiplayer was - in effect - region-locked, an EA employee guessed the decision had been made to cut down on server loads. Not quite. Army of Two senior producer Reed Schneider has told G4TV that it's actually due to the game's co-op animation routines, which just wouldn't work properly if one player had serious lag.
The game works on a deterministic or peer-to-peer network setting. Because we have all these synchronised animations and the players are always doing things together, the only way to support that kind of model right now is to use a deterministic model for networking.
He notes that it's the same deal for both platforms, not just the 360, and that they're "looking at potentially going away from a region locked setting".
'Army of Two' Won't Play Across Regions [G4TV]