The iPhone 4 and 4S always felt like a solid chunk of glass. The iPhone 5 with that extra lightness feels like a solid chunk of Oak or something of that density. That might be part of the "niceness"--its density feels more like something natural.
Apple keeps making them thinner. But at some point they'll get to a thickness and not be able to go farther. More for working with a human a hand than materials. Something as thin as a knife blade would be uncomfortable if not a dangerous thinness. And far before that it gets thin enough it would be difficult to pick up off a flat surface. That may be the deciding factor for just how thin a future iPhone could be; a pickup-from-a-desk test.
And from holding mine, I'd say that's about 1/2 the thickness of the current iPhone 5.
(And flexible displays only would work if they can become rigid after unfolding. Can you imagine typing on a display that bent and warped or folded with every touch?)