John Carmack has released the source code of the iPhone version of the famous Wolfenstein 3D video game.
You can grab it here: wolf3d_iphone_v1.0_src.zip (10Mb).
Wolfenstein 3D is written in pure C and uses OpenGL for 3D rendering. You can see how id Software uses OpenGL render calls in files such as /wolf3d/newCode/wolf/wolf_opengl.c or /wolf3d/newCode/iphone/iphone_loop.c
The zip file also contains a doc about iPhone Development by John Carmack.