Why would you do such a thing? Because you can! Attaching a DSLR or even a SLR lens to your iPhone's tiny sensor, not only will provide you with some cinematographic quality video, but it will bump your smartphone's camera capability a little bit for great unexpected results.
The DIY Approach, Take 1
Jurilog's mod gives the iPhone's camera depth of field and takes images like an old LOMO camera. Unfortunately no instructions exist on how to implement it yourself but the capabilities of the setup are amazing considering its powered by an iPhone.
Photojojo SLR Mount
$249 will buy you the option of placing a Canon or Nikon lens in front of the iPhone's sensor. Now, you will be able to do manual focusing, add filters and all with real hardware, instead of using software. You can attach wide, macro, telephoto and more with this adapter.
Owle Bubo For iPhone4
Owle Bubo is an adapter that will make the iPhone into a digital DSLR camera with the ability to use a DSLR lenses that are changable. The use of DSLR lenses would provide several benefits in addition to the sharpness of the image and soft focusing and depth of field control.
DIY iPhone-O-Scope, Take 2
Bhautik Joshi's Phone-O-Scope SLR lens mod, basically an optical coupler that allows you to mount an SLR lens onto the iPhone. The bracket is basically an iPhone case to which is mounted the magnifiers salvaged from an old CD player laser pickup, and then on top of which is a mount for the SLR lenses. That's been fashioned from cardboard tubing, PVC plumbing supplies and plenty of duct tape. Totally ghetto-fabulous.