<key>PreferPlainText</key>
<true/>
Make sure you know what you're doing when playing around in XML preference files! It worked for me under Mail 1.2.5 (v552)
[robg adds: An alternative way of adding the key is through the Terminal. Just type:
defaults write com.apple.mail PreferPlainText -bool TRUEI did this, then quit and re-launched Mail. I was hoping it would force the various spams I receive in to plaintext mode, but it made no difference in those (and I force everything else into a mono font anyway, so it's hard for me to see a difference in other emails).]