Thanks Emmett, Jim, and Mark. We're installing that tongue and groove ceilings, so now is the natural time to put down the vapor barrier, it seems. I have a picture of the 6mil vapor barrier we installed today on part of our roof if anyone is interested... As it ended up, I was able to get a piece of plastic large enough (20'x40') so that there were no seems on this face of the roof.
blog entry with tongue and groove - covered with 6 mil vapor barrier and battens