Note this issue also affects the Xbox 360 version of the game and not just the PS3 version. So both consoles have the same issue.
Update June 2013: I've confirmed the older EA game method works. To make the game work online you must get an older EA game (2008 or older) whose servers are still up. You can check which games have had their servers shut down here. http://www.ea.com/1/service-updates
Once you get the game, you must then connect online and accept the TOS. Exit out of the game and put in Mercs 2. After this it should work. I used Need for Speed Carbon and it made Mercs 2 finally connect to the EA servers without freezing so I could play online, and get the online trophies.