I have a gmail account setup as IMAP in Apple Mail and recently some new emails coming through have displayed the plain text version instead of HTML. If I check the same email through gmail.com it shows the HTML fine but back in Mail I can't even change it to HTML under View -> Message -> Next Alternative.
This solution did not work for me: defaults write com.apple.mail PreferPlainText -bool FALSE
I'm running Apple Mail 5.2 on Mac OS 10.7.3
I have just found that the same email message will show HTML when viewed in the All Mail Folder but not when viewed in the Inbox.