While I don't understand why you'd want to do it, other than because you can (and a desire to be able to say in your Flickr stream that you did), you can now attach Canon and Nikon lenses to your iPhone. Photojojo, a source of photo-accessory awesomeness, grants your wish with the $190 iPhone SLR Mount.
It's an idea that's easy to mock, especially given Photojojo's suggested photo of it with a huge Canon lens attached, but it might not be quite so laughable with a pancake prime or iPhone Telephoto lens. Even better, if you want that kind of creative flexibility, use a camera.. Or if it were a mount for a smaller Micro Four Thirds lens. But Photojojo's got a lot of cheaper, more size-compatible add-on lenses for your iPhone, like this