I've been reading up on the iPhone's GPU and 3D rendering pipeline based on the PowerVR SGX The PowerVR pipeline is based on Tile Based Deferred Rendering and advanced Hidden Surface Removal that occurs very early, prior to the actual rendering phase. Apple's developer site has some compelling information on the PowerVR platform and how it performs well as a mobile GPU.
Occlusion detection and the culling of unseen polygons ultimately means less primitives to render, which of course equals less horsepower going to effects you never see. Removing hidden surfaces too aggressively can cause weird stuff to happen, like stencil shadows popping through textures or light blooms shining through walls. But for something with limited power and screen real estate, it makes a whole lotta sense.
I can see why Carmack has had so much fun porting titles to the iPhone - it's a kit that really makes for some nice titles. And with iPhone OS 3 having more robust p2p bluetooth capabilities, this thing could turn out to be the next big gaming platform.