Speaking with Shacknews, Carmack and Mustaine both chimed in about where the Doom franchise is heading and their development experience on the iPhone, from hardware limitations to game design. It's clear from their commentary that more mobile titles are on the agenda, with DLC being a big priority. Carmack also brought out their willingness to experiment with game design, an important consideration given the difference in gaming platforms compared to 15 years ago.
The lengthy interview spans three pages and is focused primarily on id's mobile objectives. Some other projects they brought up are Doom Classic on the iPhone and Wolfenstein RPG, both geared specifically for mobile platforms. Whether you think id has a future or not, it's a great interview to read through.