> 1. Can I use a separate 78lxx type regulator supplied straight from
the main
> 24 V supply to supply the analogue part, and a switcher for the
rest, or
> will the switcher induced line noise on the power line still make my
life a
> misery?
> 2. If I use only the switcher, will I be able to get the supply
clean enough
> using RC low pass filters (bypass caps), or an LC notch filter, or
will my
> life become a still become a misery?
> 3. Any other advice/suggestions?

It all depends more or less on good PCB design. I've designed lots of
mixed signal PCB's including onboard switchers with no problems. Most
manufacturers of switcher chips like ST, Linear and National, supply
sufficient information on PCB design to succesfully apply these in
your design.

A few common important notes are :

-starpoint grounding to switchers outputcap or internal transistor
source and
 keep the high frequency current loops as small as possible.
-use a separate groundplane for the switcher
-use additional LC postfiltering for better switching noise
suppression
-lowpass frontend for the switcher to reduce switching noise to the
main
 supply
-use 'potcore' closed type inductors to reduce radiated emission
-on the PCBoard the switcher should be placed as far as possible from
 the analog circuits (amplifiers, ADC's etc.)

Futhermore I usually place a 78Lxx type 5V postregulator after the
stepdown circuit to supply the PIC and other circuits. You have to
keep the dropout voltage of linear regulators in mind when you decide
the level of outputvoltage of the switcher. Usually a couple of Volts
input to output difference should be enough.

Good luck!

Daniel...