Re: Rule of Thumb
"...Apple maintains a view that a phone's display should be easily navigated with just one thumb..."
Yes, just been assaulted by the latest 'iPhone 5 - your thumb' TV advert, so they are definitely still peddling that line. For (allegedly) incredibly smart designers of products and UIs, that is one of the dumbest self-imposed constraints imaginable, same as the 'nobody wants a 7 inch tablet' thing, despite the fact that they are flying off the shelves and lots of people (without gorilla-sized hands and arm muscles) don't want to lug around a large tablet or take steroids to be able to hold it for more than five minutes.
Of course most users will want to be able to operate their Smartphone UI with a swipe of their thumb whilst on on the move without having to juggle it from hand to hand. But users have different sized hands; my 11 year old, my wife, me, a 7 foot tall basketball player, etc. One size does not fit all; my child's dinky HTC/android is the perfect size for her, too small for me to use comfortably, and vice-versa.
Of course users (many users I suspect since bigger is so obviously always better) would actually like a bigger display on their smartphone regardless of thumb reach. Some because they want to display more content, some because it's cheaper than buying a big car with a long bonnet, and some (like me) because we're getting old and have poor eyesight so would like a scalable UI on a bigger screen SO WE CAN READ EVERYTHING IN 1" HIGH LETTERS.
The answer is of course bleedin' obvious (and therefore despite being obvious and probably a load of prior art Apple will eventually cotton on and win yet another patent battle over things so obvious that they should not be patentable). The UI and the screen do not have to be the same size (gasp!). A simple 'thumb reach' calibration exercise could be performed and the parts of the UI that need to be one-thumb operable constrained to the area reachable by the current user (I'm thinking the important parts of the home/icon screens, dialler, etc).
Clearly such an approach would require Apple and App designers to think a bit differently about how they lay out their UI's, and not every App/UI element would be suit such an approach, but I'd contend that a lot of the problem areas would be things that you would be better off operating with two hands/thumbs anyway (e.g. pinch/zoom, game controls, etc.) It would probably also necessitate a move away from fixed-sized bitmap-type UI elements in the direction of a scalable vector system, but again with increasing processor and graphics subsystem horsepower now commonplace, that has to be the obvious way to go, otherwise every single app has to be re-written every time the screen DPI or size changes!
Oh and I guess whilst you're calibrating for thumb size, you could check whether the user is "holding the phone the wrong way" thus dealing with another design issue that has cropped up before.