The log contains an entry similar to:
3/3/2011 5:59:17 PM CSession::LockOutlookLogon Error 4813 Cannot log on to the mailbox because Microsoft Outlook application is currently running on the workstation. (Server: 'Server', mailbox: '/o=Org/ou=First Administrative Group/cn=Configuration/cn=Servers/cn=Server/cn=Microsoft System Attendant', user: 'Domain\ServiceAccount')