The new cross domain planning functionality for Group Policy, RSOP Planning Mode, requires file system and Active Directory ...

The new cross domain planning functionality for Group Policy, RSOP Planning 
Mode, requires file system and Active Directory Domain Services permissions 
to be updated for existing Group Policy Objects (GPOs). You can enable this 
functionality at any time by running
"adprep.exe /domainprep /gpprep /domain ". 
This operation will cause all GPOs located in the policies folder of the 
SYSVOL to be replicated once between the AD DCs in this domain. 
Microsoft recommends reading KB Q324392, particularly if you have a large 
number of Group policy Objects.