I've actually put my iPhone inside my enlarger resting on the negative stage and printed an image from the screen onto Ilford Multigrade RC - lots of pixelation, very flat image - even with highest contrast filters. This was using the 3G with the smaller resolution screen. Now have the 4S, but I think the problem would be the same, even making 3 inch prints at the native 960 x 640. I did have an idea to make a larger 'mosaic' with thumbnail sized prints from the screen, but just wasn't happy with the results.
We don't see the pixels when we're looking at the phone display because our eyes tend to concentrate on the surface of the retina display, which actually softens the image and acts as a filter, but when focusing on the screen as a 'film plane' with the enlarger or film back, the pixels themselves (below the retina display) need to be in focus for the image to appear sharp.
Interesting idea. I've started a thread on DPUG.org about it--