According to United States Patent Application 20120147193, Apple is thinking of making the lenses part of a removable back panel so that users could swap panels to use a lens of a different focal length, or even one with a filter.
One design by inventor Richard Tsai even shows one panel with lenses at each end, meaning that the panel could just be reversed to change lenses.
For those of you who speak legalese, the abstract of the patent application reads: “A portable electronic device includes a digital imaging subsystem with a lens having an optical axis. A case encloses and supports the digital imaging subsystem in a first defined positional relationship to the case. A removable panel is coupled to the case and held in a second defined positional relationship to the case that covers the digital imaging subsystem without the removable panel being directly connected to the digital imaging subsystem. An optical component is supported by the removable panel such that the optical component is aligned with the optical axis and alters optical characteristics of the digital imaging subsystem. The device may include a power supply and an electronic control system coupled to the digital imaging subsystem and the power supply enclosed in and supported by the case. Electrical connectors may couple the power supply and the electronic control system to an electrical component on the removable panel.”