What's that up there, on top of that Nikon D90? It's a $39 hotshoe-based smartphone mount for DSLRs called the Flash Dock. While it looks like it's designed to take an iPhone an use it as an adjustable, makeshift speedlight, it's actually just a dock that holds your smartphone — no more, no less. The idea is that it'll make it convenient to use some photographer-friendly apps, like light meters, levels, depth-of-field calculators, and voice recorders while shooting. Still, sticking your phone to the top of your camera is only a bit better than holding the phone in your hand when you need it, and all DSLRs have light meters and (most) have levels, too.
The Flash Dock gets moderately more useful as a GPS unit for your camera — just use an app that records GPX data and sync it to your pictures later using your photo-management software of choice — but that's still something you should be able to pull off with your phone in your pocket, as is the case with using it to transmit photos over Wi-Fi using an Eye-Fi memory card. If you can think up a particular need for a camera mount for your phone, the Flash Dock does appear to be the best option short of going DIY; you can pick it up now directly from the maker's website for $39, or get it for a few bucks cheaper at Micro Center.