Gamespot has a
video interview with Id Software's CEO (not sure if you need an account to see it).
He talks a little about Doom III on Xbox. He basically says the Xbox is a no-brainer, because of it's architectural similarities with Doom III's target PC. He said he doesn't think the PS2 will be able to pull it off the way the game deserves, but it hasn't been ruled out, and of Gamecube he says (paraphrase) "there is some potential there, but there are a few technical questions we need to answer".
So nothing is set in stone, and he will be concentrating on making the Xbox version IF ever the game comes to the Cube.