I have seen quite alot of Windows 2003 Active Directories recently where all the Domain Controllers are Windows 2003 and all the Domain Functional Level are set to Windows 2003. However one key part has been overlooked that is the FOREST Functional Level. This is often been left at Windows 2000 mixed.
So what is the significance of this ?
Well one of the most important things you can take advantage of is LVR (Linked Value Replication). This is particularly significant when restoring Multivalued Linked attributes such as Groups for example with Forward links and Users with Back Links.
Please reference the following two articles for more information.
and the following article
This is where it is changed in Domains and Trusts. Remember this is a ONE Way Operation.