Maybe I’m missing something but it appears that Leopard Mail removed the old preference to prefer the plain text version for incoming messages over fancy HTML formatting. It still seems to be implemented, it was dutifully transferred when I updated to Leopard, but I can’t see any way to actually set it in the Preferences window.
For anyone who wants to enable this feature you should be able to do it by opening a Terminal window then type the following command followed by a RETURN:
defaults write com.apple.mail PreferPlainText -int 1
or to turn it OFF:
defaults write com.apple.mail PreferPlainText -int 0
Hope this helps.