Carmack said on an interview that they're thinking about releasing some M-rated games for the DS but they're afraid that there's no market for that.. so, they're seeing how the sales will go for the M-rated titles out there to decide that..
I don't think the market is small, since lot's of "mature people" use have a DS and like these kind of games.. and obviously, kids like M-rated games! And most people sell it to them! I loved games like Doom, Wolfenstein and Mortal Kombat when I was 8!
I don't think we will see a GTA game for the DS, a 3D version would be too different (I don't think the DS can handle a map that big, with so much diversity of people, cars, etc) and it would end up disappointing most GTA fans.. and I don't think anyone would be interested in a new 2D version.. specially when the PSP has 2 GTA games that are almost identical to the PS2/Xbox/PC versions.