In a perfect world this would only change non-html emails to a monospace font. I think it works better than any other script/extension that I've tried... Additionally this makes the compose feature use a monospace font (it doesn't affect rich formatting). For other gmail styles I've made see Gmail: sidebar/fixed height and Hide Gmail/Google Appbar.
Not working perfectly for gmail 2.0 yet, but trying...
Works for me with the new version, unlike GMail: monospace font in non-HTML mails, v2, which claims to work with the new version.
Doesn't work with Gmail 2.
Very useful style, takes you back to old good times when all e-mails were clean and could carry ASCII-arts :)