IMPORTANT NOTE
In order to play Mercenaries 2 online, on either PS3 or 360, one has to have agreed to the TOS in Army of Two's multiplayer. Yes, you read that right. Something is screwed up with Mercs 2's TOS, and in attempting to access the multiplayer component, the game freezes. EA's servers need to see you've agreed to the TOS from a game of the same TOS phrasing as Mercs 2, and Army of Two is the only one left that anyone online is aware of. I have faced this problem personally and this worked for me, and it also worked for a friend whom I lent Army of Two to in order to do this specific thing. Look it up if you are curious, there are many, many forum posts all over the net about the issue.
If you think you'll ever play Mercs 2 online, grab Army of Two, agree to the TOS, and then return the game or keep it BEFORE AUGUST 11th!