I came across this Kickstarter project via PetaPixel this afternoon and was intrigued - it was something I thought of and could have used just yesterday afternoon. The use case for this product isn't what I had in mind, though.
We were at a sporting event for our youngest daughter and I wanted to capture both still pictures and video at the same time. My DSLR (Nikon D300) doesn't capture video, but an iPhone 4 does a fine job. Unfortunately, I can't do both at once. I thought "it'd be kinda cool to have a little clip that could hold the phone under the camera - either on the hot shoe or tripod mount." I made a mental note (the kind I forget quickly) to look online later.
Seeing this project today jogged my memory, though the project founder has a different idea - mount the iPhone under your camera and show photos or video to keep babies and kids looking right at the camera. Also a good idea. It looks like a pretty sturdy piece of gear - though quite pricey at a $99 expected price point. It's definitely overkill for my use case, but it's the first option I've come across that mounts the phone under the lens rather than up on the hot shoe.
If you can spare the hot shoe, there are some cheaper options such as this one shown at TUAW last month. There's another commercial option from Spigen (reviewed at iLounge)and at least one DIY option as well. Each of these is pitched as an easy way to either capture video at the same time, use the iPhone GPS to geotag photos (to pair up later), or to take advantage of photography-related apps.