With the announcement of the didtheyreadit.com email tracking, I figured it might be a good idea to submit an easy way to fix this. To disable the tracking service, open up Terminal from Utilities and just do the following:
sudo echo "127.0.0.1 didtheyreadit.com" >> /etc/hosts
Also, the way this service works by appending an invisible image into the originator's email. The image pulls from didtheyreadit.com, so you could also filter on didtheyreadit.com in the message (i.e. set up a new rule looking for that string). It would be nice to reply to someone asking what the problem is...
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040523221845391