The short version is that Exchange Server 2003 SP2 introduced changes in the security model for the delegate-setting process which, combined with updates on the client end, result in the following error when a user attempts to modify delegates:
The Delegates settings were not saved correctly. Unable to activate send-on-behalf-of list. You do not have sufficient permission to perform this operation on this object.
This error does not occur for all mailbox, and we've found neither rhyme nor reason for when it does, but there is a fix.