Upgrade the CDO.DLL on your BlackBerry server to version 6.0.6487.2 ... if your users that are having a problem are running Outlook 2003 ... you need at minimum, the CDO version I indicated above in order for wireless calendar synch to work as designed.
Microsoft introduced a new calendar attribute with Outlook 2003 and the old CDO doesnt know how to read the attribute ... thus causing wirless calendar synch to choke with users on Outlook 2003.
This should solve your problem, and no, you do not need to apply this to all of your Exchange servers ... just the BlackBerry server will be enough to solve the issue, if indeed it is Outlook 2003 users that are having the problem .... if your Outlook 2000 users are having the issue ... then time to beat up RIM support some more