nrc, have you run into
Bug 25828 - GroupWise: Shared addressbooks get migrated to user's Contacts ?
The more I look at it, the more worried I get. Picture two users each of whom has aliases or contact groups like "Mom," "Friends," "Advisees," "Family," "Clients," "Prospects." In the GW addressbook, it is clear whose is whose, but when migrated to Zimbra, it's a crapshoot.