You can always rig an existing camera lens to your phone, and the progress on the iPhone DSLR project is amazing, but this looks to be a more portable solution. A fisheye lens is available for $25 and a wide angle/macro lens is $20. You can get both together for $40.
Be warned that the lens does cover the flash on the iPhone 4. The lens can be used with a case as long as the case is tight against the phone. The ring itself is attached to your phone with adhesive and some will be reluctant to mar the surface of their phone with it, but using a case may offer a solution if you're willing to take the risk of possible light leaks.