GameSpy took the chance and made a nice three page interview with him. You get a lot of interesting informations about DOOM 3 and the engine plus Quake IV and more.

GameSpy: I am surprised that you do not see Quake IV using this next generation engine.
John Carmack: Wolfenstein worked out so spectacularly well. Inside id there is a group that really, really does not want to do another sequel. Our next game is not going to be a DOOM, Quake, or Wolfenstein sequel, it's going to be something new and that is a foregone conclusion.
We knew that we were going to license that out and let someone do it.
The work that was done with Wolfenstein, in the end it turned out great, of course, everybody loves it, but it was painful during the development. At one point we had every programmer at id working, trying to stabilize the code base to make things okay for shipping.
But still, we looked at that as a success and we are hoping to replicate that success with Raven doing Quake IV. Raven has always been our lead engine adopter. It was a foregone conclusion that Raven would get the Doom engine early, the only question was what they were going to work on.
Prepare for more long answers like this and get the full read over here.