A film making firm took a Canon 5D Mark II and an iPhone 4S, placed them on a rig for simultaneous shooting and then put the resulting shot side by side for your very own analysis. The matching of the exposure, frame rate (30 fps), shutter speed and picture style was made sure by them.
As it can be seen, the results are amazing, especially for the iPhone 4S. The image taken with iPhone 4S is a bit stove than the one taken with the Canon, but indeed it requires no more evidence to prove how good the video capabilities of the 4S really are.
On maximizing the video on a 27″ iMac, the difference between the 21.1 Megapixel sensor on the Canon and the 8 Megapixel sensor on the iPhone 4S could easily be seen as some details show up a little blurry on the iPhone footage.