We run OCS/Communicator 2007 and it has run fine for some time now. Yesterday I moved many users from Exchange 2003 to Exchange 2010 and now when they login to their computer they get a Communicator login prompt. They can cancel out and it still works fine. I'm assuming it is the integration of Communicator with Outlook for calendar and status. These users are all internal. We use OCS 2007 and Outlook 2007.