I guess we tend to major on the chips, but I quite admire the way the use of plastic materials and mouldings has also come on in leaps and bounds. Dave mentions it briefly, but there's a shed load of development in the design and plastic moulding tools, too, I'd guess.
OK, so now fast-forward 20 years... multilayer devices "printed" by 3D printers that don't have screws, because you can't dismantle them? A mind-port rather than a keyboard, because you only need to picture the person you want to contact, and the phone does the rest? Low power electronics and batteries that last the life of the phone, so don't need recharging? Predictive speech algorithms that finish your sentences? "Hello darling, I'm going to be Late/Famous/knackered/Rich/Dead..." hmm, some further work required, there!