It all became possible thanks to a group of fanatics who started working on Mafia 2 MP client, but shortly after the initial development phase the project was halted for reason, they have mentioned here.
Nonetheless it didn’t stop guys at ModMyWhat to resume where they left off. They are actively testing the Mafia 2 multiplayer client and have released few videos and screenshots of the mod in action, which you can see below.
Mafia 2 Multiplayer Mod Test#1
Mafia 2 Multiplayer Mod Test#2
Mafia 2 Multiplayer Mod Test#3
Not much details about Mafia 2 Multiplayer Mod has been released yet as its still in Alpha phase of development, but you can check out the development progress here.
We have already seen working GTA San Andreas and GTA IV multiplayer mods, but hey, its Mafia 2, and it would be cool to have a Mafia 2 MP mod also. I wish them good luck and hope they make one stable client.