Cellphones are getting smaller and lighter -- some Japanese i-mode phones weigh in at less than 50 grams. This is light enough that it's not inconceivable to actually wear the entire thing on your head, like those goofy radio-headphones from the 70's. So why not put a spring clip in the back of a cellphone that will go over the top of your head, and hold it in the right position? It could even retract like a tape measure, so would only marginally affect the form factor of the phone.