Why don't accepted event invitations through MS Exchange show up in iCal?
Some time ago I wrote a script to fix the problem that Exchange had with incorrect time zone data. The script will import Exchange events into iCal. There are several post on my blog concerning the script and how it functions.

Go to my blog, Surgical Diversions, and search for iCal. You'll see at least 3 posts regarding the script.

This solution is for those without an Exchange account and/or who don't want to set one up. The simplest solution is to create an Exchange account within iCal.