Borderlands has now gotten rid of GameSpy and moved to Steamworks
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Borderlands has finally made the move to Steamworks and ditched the now defunct GameSpy servers.
This means that you can now play the game in co-op again, following the GameSpy shutdown, which left many players unable to enjoy the game in co-op.
The developer posted an update on the Steam forums, announcing that the migration is now complete.
Thanks, Destructoid.