Sean,

> Does anyone have any info on the thermal conductivity of PCB material?
> Roughly,does it conduct heat well or insulate it?

Roughly, it conducts.  Much better than air, but not as well as metal.

Check App Notes from surface-mount power device manufacturers.
They deal with this to calculate power handling for practical circuits.

I have International Rectifiers HEXFET book, they give data for
several substrates.  Ugly mixed units : Watt-in./ square inch - deg. C

no data for paper-reinforced phenolic.
FR-4 / G-10 (glass reinforced epoxy): 0.0072
Alumina:        (ceramic)                    0.45

Note that this is for the SUBSTRATE.  The copper foil is a MUCH
better conductor of heat, so _the amount of copper area you can
provide as a "heat sink" matters more than the substrate_ unless you
need to pull the heat through the PCB.  There is a graph in this app
note that shows that the thermal resistance of the PCB from
package to ambient decreases linearly with pad area.

If you take apart a cell phone, or car stereo of recent vintage you
will see that they surface mount power components on the PCB, and
they do leave large (usually ground-plane) copper areas.  Then the
ground plane conducts to the case or heat sink, and its pressed
against the part packages on their top sides.

So, I'd surf for some app-notes from IR,Motorola, Supertex,
Siliconix, Harris, etc. for power FETs and power ICs and check out
their data.  If you have a specific part that you are looking for a
packaging solution for, ask the manufacturer about app notes.

Hope this helps.
------------
Barry King, KA1NLH
Engineering Manager
NRG Systems "Measuring the Wind's Energy"
Hinesburg, Vermont, USA
www.nrgsystems.com