man I can't see Carmack giving in to a console like that, especially Microsofts. He's said before that iD will always be 100% PC. Not only that but as we know the XBox graphics is based on DX, you make a game for XBox you have to work with DX. When was the last time you saw a DX game come out of iD? That's right - never. I think that rumor is total BS.
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you just never know though. If it turns out to be true, the huge amount of respect I had for Carmack and iD will be gone.