Joe Pfeiffer wrote in
:

I've got a 25 year old house, whose light switches I'm converting over
to a powerline protocol called INSTEON (see www.smarthome.com; for
purposes of this question, it's very similar to the old X10 protocol).

Two of the bedrooms are on one circuit breaker (the whole rooms:
lights, outlets, the whole shebang). Each room has a single switch
controlling the lights in that room. I pulled a light switch out this
evening, planning to replace it, when I saw there was no neutral in
the junction box (the INSTEON switches require both hot and neutral).
At first I expected to find neutral in the wall behind the box (I was
guessing they had, for some reason, routed hot through the box and to
the switch while leaving neutral unbroken), but then realized that
what went in the box was a single piece of three-conductor wire, with
black used for hot, white for switched, and ground not connected.

So... before I start digging the junction box out, what are my odds
of finding neutral somewhere in the vicinity? More generally, I'm
having a hard time imagining why someone would wire the ceiling light
fixture and not have power and neutral come from the same place! So,
before I get myself in trouble, how did they likely wire it?

Thanks in advance,

It's called a switch leg. Power should actually come in on the white
and leave the switch on the black. There is no neutral in the box or
probably anywhere near. This is normal and probably half the light
switches in the world are wired this way.

Code prohibits you finding a neutral from some other location. All
circuit conductors should come from the same cable.

You could replace the switch leg with a piece of 3 wire (don't count the
ground in a cable with uninsulated ground) from the light to the switch
and abandon the 2 wire you have to gain a neutral for your switch.