So Vyshemirsky found another solution to this problem called at http://ww.snerdware.com/groupcal and they have another than does the same for contacts to the Address Book. Although Judi found it buggy and it deleted appointments in Exchange and iCal (so backup before trying any of this!). And some think they are out of business.
There are real issues like Justin point out and Fragens has a fix for. Outlook 2003 and earlier uses non-standard time formats like (GMT+02.00) Harare/Pretoria whereas Apple uses Africa/Johannesburg and so when iCal gets a calendar request from Exchange, it simply adds two hours to the event start time becaue it can’t deal with GMT+2. He wrote a script you run on your computer that runs in Mac Mail, it intercepts the message and then fixes the dates. Clever guy!