This time, I was getting "Unable to load any usable iso8859 font"
errors anytime I'd try to start xterm. I searched the FAQs and the
web, but was unable to find any suggestions that led to a solution. At
one point, while trying to fix the problem, I also got the "could not
open default font 'fixed'" error, but none of the solutions listed in
the FAQ were applicable (either the problem described was obviously
not a match, or the solution, like re-installing all fonts, had
already been tried).
As a last resort, I used the Cygwin setup log to figure out which
packages had been updated, and rolled them all back to the versions I
was previously using. Now my X applications are working fine again, so
something in one of the updated packages definitely triggered this
problem.