I know there have been a lot of Entourage «» Mail «» iCal «» Applescript hints, but I think this one is a bit different. I use my Entourage calendar as a pseudo "string around my finger" for quick reminder notes to myself for things that I need to get done later in the day. Sometimes, however, I am at a different computer, and I normally just send myself a reminder email, which doesn't ever get into my calendar.
I decided to write a rule and an applescript to place mail messages whose subject start with "[calendar] " (no quotes) into the Entourage calendar automatically, and to immediately remind myself. It turns out that Microsoft already has a "Create Event from Message Reminder" in the script menu, so I just modified it accordingly.
Create the AppleScript in Script Editor, and save it to /Users » username » Documents » Microsoft User Data » Entourage Script Menu Items. Then set up a rule in Entourage as follows:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070304180334380