Am I the only one who interpreted Carmack's response as primarily saying "new games cost a heck of a lot more to port than old games, and as far as we can see the market has grown but not enough for a port to make sense" ?
The graphics subsystems are already being addressed; I think one could make a pretty case that even the open source drivers will reach an appropriate level in less time than a port would require, and an iD engine is one of those apps that everyone will be sure to support properly.
The big unknown that I see is still market size; which is a combination of (a) widely varying numbers for Linux market share, (b) an apparent bias towards low end systems (eg netbooks) which may not be able to run Rage usefully, and (c) not enough market data for sales of Linux-specific SKUs to make a case for how much additional sales will result from providing a Linux client vs hoping Wine will suffice.
The only really credible market share numbers suggest maybe 1% market share for Linux vs 8% for MacOS, although there are all kinds of seemingly reasonable numbers around (from individual distros) which suggest a much larger market share for Linux. It seems to me that the most useful thing we could do here is to paint a consistent, credible picture for game developers of the market potential for native Linux clients.
I don't know if the required data even exists; I can tell you that if it does it's pretty darned hard to find.