the Photojojo Store has a $249 solution in the form of a special mount for the Nikon lenses I already have:
On Thursday the Photojojo Store released the iPhone SLR Mount, a hardware attachment that allows you to use your fancy Canon EOS or Nikon SLR lenses while shooting pictures with your iPhone. It costs $249 for the iPhone 4 version and $190 if you get it for the iPhone 3GS or iPhone 3G models.I want one. But the whole grad school tuition thingee will probably get in the way of that dream.
"The iPhone4 takes pretty decent photos and gorgeous video, but its limitation is the lens built into it that can't go wide or give you shallow depth of field," said Amit Gupta, founder of Photojojo who worked with a company in Hong Kong to get this product made. "But if you use an SLR lens you can focus on a person or subject and blur out the background for that shallow depth of field effect."
It also allows you to zoom in without compromising the resolution.