> 1. Why do you need several different voltages?

Different circuits need different voltages.  It's why the connector you plug 
from the PSU to your motherboard has 20-odd wires and not just 2 big fat 
ones.  In an iPhone I imagine you'd at least need different voltages for the 
digital circuits, the audio circuits, the display and the RF ciruits.

> 2. I'm not aware of any way to change the voltage of a DC circuit.

This is often used for battery powered devices as it's very efficient:

http://en.wikipedia.org/wiki/Charge_pump

> Mmm, interesting. (And ridiculously complicated.) I wonder how much of 
> this stuff is to do with electromagnetic radiation? (You don't want EM 
> interference stopping your motherboard working, and you don't want the 
> board to radiate EM either...)

I assume they must pass certain legal test standards for EM compatibility, 
but there are products that require stricter controls, especially ones that 
are used near other RF equipment where interference is unwanted (eg medical 
equipment, circuits in radio receivers, on planes, near to any 
safety-critical device etc).

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