When checking my sent mails in Apple Mail I noticed that all images had been replaced by a 'Missing Plugin' notice. I traced this to an issue with the Java plugin unavailable to be displayed in Mail.
The solution was to reset the Java preferences for the top (web browser) items on the 'General' tab of the Java Preferences application to standard. Make sure the boxes are checked for those items and click 'Restore Defaults.' This app can be found in /Applications/Utilities/.
[crarko adds: I haven't tested this one.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20100623230636227