LinkLetter is a very simple AppleScript helps users link emails with any other application by finding the message location on your hard drive. For example, use it to link emails to iCal To-Dos or events.
Just select a message in Mail and run LinkLetter. Once you hear the beep, paste the link to the email into (for example) the URL field in iCal.
You can open the created links either with "Open location" in iCal or from any app with the Services > Open URL command.
Just copy the script to Library/Scripts inside your home folder to access it from the scripts menu.