Bill, Jim and mnpalms all know a lot more than I do about overwintering palms.
I have a single datum about using pipe heating cable as the only heat source in an enclosure.
I wrapped 30 watts worth around the trunk and spear of my large Butia, covered that in burlap, and put a 4' x 4' x 4' plastic "palm hut" with insulation on the top over it. I put a 5 gallon water jug inside to buffer temp. fluctuations.
Here is a photo from February:

<a href=" http://picasaweb.google.com/lh/photo/CwNQ0eYLtHOzdx8nkmXVEg?feat=embedwebsit e"><img src=" " /></a>

In my opinion, it worked great. When outside air temps got to 4oF, it was upper single digits inside. That should have done in the leaves, but they came out great (except where they contacted the plastic walls; those were toast).

I simplistically think of the trunk and spear as water pipes and so pipe heating cable makes sense to me.

I think some of the confusion comes from using the same phrase, "heat cable", to refer to both the seed germination heat cable Bill uses under his bananas and the pipe heating cable I wrapped around trunks/spears.
--Erik