one of the most impressive 3D apps out, Magic Donut really shows off how much computing power is available in your iPhone.
3,500 vertices are recalculated every frame at 30 frames per second (3GS), to form a dynamic, constantly changing torus (donut) complete with shading and touch-interaction. you can also change the coloring, shading style, and parameters defining the toroid's shape.
for the technically-minded,
this is using the fixed-function pipeline, no shaders. it's also not using GL lights, but rather calculating the shading itself in native Objective C. to obtain the shape, a base torus is stored and each frame a distorted one is generated off of it through the magic of sines and cosines. all shading and coloring is just vertex colors; there are no textures involved. vertex normals are calculated twice per iteration, once to help with the distortion, and again for shading.