Migrated from Exchange 2003 to Exchange 2007. I have a mix of clients in Outlook 2003 and 2007 - and I'm not convinced anyone is running the update tool.

1. Granted my Administrator account Send-As access:
   Get-mailboxserver EXCHANGE | add-adpermission –user Administrator -accessrights GenericRead, GenericWrite -extendedrights Send-As, Receive-As, ms-Exch-Store-Admin
   (Where EXCHANGE == Exchange server name, Administrator == user to grant access to.)
2. Dismount and mount mailbox store (during maintenance, of course.)
3. I notified all users to patch their systems  (OS and Outlook patches).  Print paper copies of calendar items.  Deploy patches as necessary.
1. OS patches http://support.microsoft.com/kb/931836/en-us
2. Office patches.
4. Make sure server OS patches are installed.
5. Find a machine with Outlook 2003/2007 on it, NOT the Exchange server.
   1. Download TZMove http://www.microsoft.com/downloads/details.aspx?amp;displaylang=en&familyid=E343A233-B9C8-4652-9DD8-AE0F1AF62568&displaylang=en
   2. Download the Exchange Calendar Update Tool (just a wrapper) http://www.microsoft.com/downloads/details.aspx?amp;amp;displaylang=en&familyid=a9336886-4b28-4010-9416-36d38429438d&displaylang=en
6. Run MsExTmzCfg.exe, answer questions, which will generate a BAT file for you.
7. Run the BAT file to update all appointments on servers.  If you get errors, check the INI file to make sure the /O= line is updated with the appropriate server path in the AD.

As an aside, if you're on Exchange 2003 SP2, you'll need to patch your Exchange server as well in step 4: http://support.microsoft.com/?kbid=926666