Ok I have the original 20 GB launch PS3.
Staying signed in to PSN, I copied my saved game and profile data to a flash drive, and then deleted my save game from the HDD.
Then started a new game, went straight to multiplayer, and it worked.
Exited the game, copied the save data back from my flash drive to my HDD, loaded GTA and I'm now back on multiplayer.
Hope this helps some of you let me know if it does!!