Man. I have been a HUGE fan of Craig Mullins’ artwork since I first played Marathon, the classic FPS on my Mac back in college. He did all of the most influential cocnept art for Bungie’s games. His website is a must-see, loaded with his amazing digital paintings. I haven’t seen a whole lot of illustrators’ tutorials around recently, which I think is a shame, since they are so valuable. I found a few tutorials Craig Mullins did on the web, and they are fascinating:
http://www.gfxartist.com/features/tutorials/7731
http://www.gfxartist.com/features/tutorials/7840
http://www.gfxartist.com/features/tutorials/974
http://www.gfxartist.com/features/tutorials/7863