I'm on Lion. I have a client who sends meeting invites from her Outlook account (I think also Exchange server). If I click on the .ics attachment it goes into iCal and I can accept/decline/maybe. If she sends a notice of a cancellation I can't do anything to it. I have to go into iCal and manually delete the meeting (of course this is doable) but if it's a recurring meeting, it's not a delete but rather a decline.
Is there something I can do to fix this? Maybe she's doign something weird on her side? (Totally possible.)