20 years ago -- heck, even 10 years ago -- it was widely agreed that the best camera for street photography was a Leica rangefinder.
This is the camera that made the careers of photojournalists, street photographers, fine art photographers et al, because while it came with limitations -- BOATLOADS of them -- it was also a tool optimized for capturing moments, which is what a camera is supposed to do. It stood in stark contrast to the weapons-of-choice of the fashion/portrait photographers (medium format SLRs/TLRs), sports photographers (fast 35mm SLRs) and landscape photographers (huge large format view and field cameras): It didn't have a lot of fancy features to get in your way, it was just one focal length of great optics coupled to a slim profile that you could set to f/8 and just look for shots.
The iPhone 4S is slimmer, faster and slightly better than that old Leica for the same purpose. The interface is better than that of your Leica M8s and Sigma DP-Xs and, yes, even better than the el-cheapo cameras that plague the market. Point. (Click to focus if you need to.) Press the button. Email the thing. No dials, no modes, no craziness, it's exactly what a non-photographer needs and, like the Leica, quite capable of taking great photos.
I went out on a shoot the other day with my wife and baby. I brought the 5D mark II, determined to get a great shot at a nearby trail I have been to a thousand times and never got a great shot at. I took 500 shots. But the shot of the day was taken with her iPhone 4s: me in the foreground, perched precariously on a slope, framing a waterfall in my viewfinder, with the kid hanging from her backpack carrier, looking under my raised arm at the falling water.