Fortunately, I found a post on a forum that clued me into the problem,
or I probably never would have solved this...

Somehow session cookies in the temp directory that needed to be deleted
were causing a problem.  I executed a 'rm -f /tmp/ruby_sess.*' and my
Rails app ran fine with or without the www subdomain.  I can't say if
this is a bug in Rails or a Ruby library, but I hoped it gets fixed, as
it took quite a long time to figure it out.