No problem in 3.x... Unity has special super-secret filter flags we pass in that Microsoft supports with Ex2K and Ex5.5 SP4 and later for finding hidden objects in the directory. Prior to SP4 if it was hidden in the directory we couldn't find it via LDAP queries.
Just hide the user from the address book in Exchange. In Ex5.5 this is on the "advanced" tab in the Exchange administrator in Ex2K this is on the "Advanced" tab in Active Directory Users and Computers (remember to check the View | Advanced options or this tab wont show up).
