There are so many apps for the iPhone that you can find something to do just about anything you can think up. If you are a photographer there are bunches of apps that you can get on your iPhone that will turn your device into a light meter and a lot more. I can see the need to mount the iPhone to your camera if it is your light meter.
A geek named Nigel saw the need to mount his iPhone to a DSLR flash hotshoe and set about creating his own gadget to do it. He used hobby plastic, measured and cut to fit into the hotshoe and to fit the iPhone. Once parts were cut, he glued them together until he had the finished product.
He notes that he made the plastic holder tabs large enough that they blocked some of the screen of the iPhone so he felt better about the phone begin secure. The finished product may look a bit unsophisticated, but it works undeniably well. You can build your own version of this following Nigel's DIY steps here.