BN writes:
 > If you have an older house with lath and plaster, you probably have an
 > old style chimney that's not being used. Maybe it's even roofed over
 > like at my parents house. Run wire in it from the basement to each of
 > the rooms on the first floor, then up to the attic to all of the rooms
 > on the second floor from the top down.

Thanks.  That would probably work for many old houses.  Turns out that
my chimney IS in use.  Worse, there's no space around it and there's
no plumbing to the third floor.  So anything I was to do looks like it
would be drastic.  I had a wiring contractor come and take a look.

Back to 802.11b!

This seems like a pretty good time to say 

Many, many, thanks to all of you for the information and the
thoughtful and helpful words.  

Best,
R