Of all the features on digital cameras that I wish were standard on camera phones, an optical zoom lens is at or near the top of the list. (Also at the top is manual exposure controls to increase or decrease exposure in, say, 0.5 increments of +2/-2 or, better, +3/-3.)
From MobileWhack I see that if you have an Apple iPhone, you can get a 6 x 18 zoom attachment for $14.77 from DealExtreme — if you’re willing to carry a rather monstrous lens (see below). The price certainly is reasonable, especially if the quality is good. But even if the quality is great, do you really want to carry this lens?
However, I think this could be a great prop for a television show or movie. The characters on several TV shows, such as CBS’ “Moonlight” and NBC’s “Chuck,” use iPhones. Both shows are techie-oriented, so using a zoom lens would be very appropriate.