I am trying to find information on the power supply circuitry inside
an iPAQ.
Specifically, is there a voltage regulator that sits between the
battery and
the processor/memory system, and if so, what is the manufacturer part
number.
Ideally, would like to see a circuit diagram of the entire power
supply circuitry of the iPAQ.

Since it's a proprietary design, it's doubtful they would release any
information about the voltage regulator.

But one would think that the DC/DC converter is a separate part that
is mounted on the IPAQ board. It shouldn't be impossible to figure out
which company is supplying Compaq with DC/DC converter chips (like we
know for instance that it uses an Intel StrongARM as its CPU).