I launched iCal today, for the first time after upgrading to Mac OS X 10.4, and I noticed something interesting. There was a notification that was asking me if I wanted to accept a meeting.
We use Exchange at work, and iCal looked at the mailbox in Mail and noticed that one of the messages was a meeting request. It parsed that and presented this notification. When I accepted it, it put it in the calendar that I chose and it sent an email to the person who scheduled the meeting.
I then looked at the preference settings for iCal, and I saw that this functionality is optional.
The problem that I have with this implementation is that the invitation does not get moved to the Exchange "Calendar" folder. This means that the meeting does not appear to Microsoft Outlook itself. It seems like it would be better to either leave the message in the mailbox, or to move it to the Calendar folder.