You may encounter an issue with Exchange Server 2007 organisations that have been migrated from previous Exchange versions where the Default Offline Address List cannot be enabled for web distribution, even after the OAB has been moved to an Exchange Server 2007 server.
In the Exchange Management Console the check box to enable web distribution is greyed out.
To resolve this issue use the Set-OfflineAddressBook cmdlet to apply the mandatory properties to the OAB object.
[PS] C:\>Set-OfflineAddressBook "Default Offline Address List" -ApplyMandatoryProperties
When viewed in the Exchange Management Console the check box is now available.
You can now tick the box and add OAB virtual directories for web distribution.