Turn HTML off completely in Mail.app

I hate HTML mail. Apple's Mail has a preference that allows you to turn on plain text emails for sending but there's nothing obvious that let's you specify that you always want to read mail using plain text. There's a hidden preference that can be set by dropping the following into a shell:

defaults write com.apple.mail PreferPlainText -bool TRUE

You should now get all mail as boring old readable plain text.