We are witnessing an onslaught of Android phones at CES. I think it poses a challenge to iPhone in this sense. With all phones (but especially the smart phones), the buyer/user invests time and money in learning/buying the software. Even if iOS stays competitive, as a user I might be concerned to invest in iPhone knowing that my hardware choices will always be very limited. Mac has always been in this situation (and suffered as a result) but with just one phone model (vs dozens, soon hundreds of Android phones) this might be very disconcerting to many.