Is the Steam version of Farming Simulator 2011 compatible with the US retail(disk) version?

[Apolonio The Prickly Spider Guy]

I want to see if that is so because I want a person from my school and my cousins to buy the game on Steam so they can play mulitplayer with me even though I only have the American physical non-steam version of the game I bought from Wal-mart without even knowing that it was on Steam. Oh well, I think it's better to actually own the game on a disk, because what would happen if I lost my account? I would lose all of those games I payed for. So please help.

[Alyx]

i think not