Although the iPhone camera is good (and continues to get better), it still can't provide the flexibility that a camera with interchangeable lenses can. Apple is apparently aware of this, as they've filed a patent for an iPhone with swappable glass.
The lens would be part of a removable back panel, and according to Engadget, it's possible that there could be multiple lenses available on a single panel (though that seems like it would be somewhat ugly and very unlike Apple). Is the era of the interchangable cameraphone lenses upon us? [USPTO via Engadget]