SNAP! PRO makes iPhone cases with interchangeable lenses. The lenses vary from tele to fisheye. Hell, they have even a CPL filter. 🙂
The case is build like a tank and image quality is very ok.
The case is tripod ready.
Price start 59 USD for the basic package to 289 USD for the all-in-one package.
More on SNAP! PRO website. (images taken from their shopping page)
More info about the SNAP! PRO: The PHOBLOGRAPHER tested the set and it got 4 stars.