I have a similar situation. I can accept an Outlook invitation without any problems. The problem occurs when a change is sent to the original invitation (time or date change, etc). Opening the ics file does nothing. The meeting in ical doesn't change. I may receive a message that says "Go offline or revert to server", but not all the time. The only way this is fixed is if we ask the person who sent the original invitation to delete it, which will delete it from ical (why does this work?) and then send a brand new meeting invitation with the new info. Anyone else see this issue? This happens on multiple machines, iMac 27 running Lion and MBP 15 running Lion.