I just installed exchanged server 2010 and some users have Outlook 2007 and some have 2010. Any user running Outlook 2007 can't accept the calendar invite form the 2010 user. They either get a blank invitation sent, or the calendar is simply blank and says you do not have permissions. All permissions are set to reviewer and I've also tried different settings on the clients (Both the sender and Receiver) machine with no luck. The 2010 Outlook user does not have a problem.