Same thing happened with Diablo 3. After announce of the Diablo 3 and revealing design of the game, someone decided to quicly make a game that looks exaclly like diablo 3. Now we have the "Torchlight".
As i remember Rage was announced way before Borderlands, go figure...
That's a different situation. Runic Games, makers of Torchlight, is made up of many Blizzard veterans and they intended to make a Diablo style game. Many of them worked on Diablo itself.
Edit: I did the research, Borderlands was announced a few months before Rage in 2007