Does enabling alpha blending affect the performance (fillrate) on the iPhone?

Question

In particular on the 2G and 3G models?

Answer 1

Yes. Absolutely. (But it's still not as expensive as alpha testing.)

From the OpenGL ES Programming Guide (for the non-3GS PowerVR SGX):

Another advantage of deferred rendering is that it allows the GPU to perform hidden surface removal before fragments are processed. Pixels that are not visible are discarded without sampling textures or performing fragment processing, significantly reducing the calculations that the GPU must perform to render the scene. To gain the most benefit from this feature, you should try to draw as much of the scene with opaque content as possible and minimize use of blending, alpha testing, and the discard instruction in GLSL shaders. Because the hardware performs hidden surface removal, it is not necessary for your application to sort its geometry from front to back.