On Vimeo, Robino Films did this "fair" test between the iPhone 4S and the Canon 5D MK II. I've been shooting video and stills with the iPhone 4S for a couple of days, and I've been pretty blown away by the image quality. Was planning to do a side-by-side analysis like this myself, but this guy beat me to it, and the results are visually informative. Snip:
I made a little rig that allowed me to shoot both cameras at the same time side by side. All scenes are perfectly synced together so you can pause and scrutinize the frames! See photo of the makeshift rig in the photo area. Exposure, shutter speed, frame rate and picture style were matched as close as possible between the two cameras. (I used the Zacuto Z-finder to help me adjust exposure for both iPhone and 5D)
This test shows that the tiny F2.4 lens and sensor on the iPhone are pretty nice. It even got a little depth of field!
I did not overlay the exact ISO and Stops 5D settings per shot. It was too much to keep track for this simple test. To be fair I matched 5D exposure to the iPhone so no "Cinestyle" / 24p here.