Thanks Benny. I think I'm a bit closer to finding the culprit. Following your suggestion, I uninstalled all the gems I could find that were not needed, including Mail, Actionmailer, Rails, etc. No change -- same error message.
But you said "... you have something in your application that redefines Mail.", so I started looking through all the gems for any that defined a Mail class. The radiant-mailer-extension gem has one that includes a valid_config? method and then there is this one that I'm guessing supersedes it: /usr/local/lib/ruby/gems/1.8/gems/radiant-0.9.1/vendor/rails/actionmailer/lib/action_mailer/vendor/tmail-1.2.7/tmail/mail.rb The tmail/mail.rb defines a Mail class without any valid_config? method. Now what? -- marshal