My company uses Exchange as our mail and calendaring backend, and we rely heavily on Exchange - Outlook -Entourage's ability to send meeting notifications to invitees. I hate Entourage, so I use Apple's Mail and iCal. The only problem is that meeting invites do not always get parsed properly by Mail.
Sometimes they show up as standard .ics files that I can double click to have them imported into iCal. Other times they show up as links to somewhere on the OWA (Outlook Web Access) server that don't resolve. Inspecting the email as raw source shows that the ICS data is actually in the email, it just isn't showing up right. So I wrote an AppleScript that will extract ICS data from an email, save it to a temp file, import it into iCal, then remove the temp file.
I set this script as the action for a Mail rule of the form:
Content-class -- Contains -- urn:content-classes:calendarmessage
Note that the Content-class header is not in the default list of headers on which you can set a rule action, but you can add it.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060821073102694