Have you always wanted to use your pro-grade Canon or Nikon lenses on your beloved iPhone 4? There's now a mount that allows just that. It's called the iPhone SLR Mount and can be found at Photojojo. More specifically, there are two mount adapters available - one for the Canon EF-mount and one for the Nikon F-mount. In order to adjust aperture, the lens needs to have an aperture ring. In other words, newer digital lenses are out of luck.
Furthermore, there are two loopholes on each end of the case; so with a strap, you can hang it around your neck just like a DSLR.
The contraption can be had for US$249.