Hello,
I believe we have a document somewhere that talks about this. :) Backing up a Windows 2003 Active Directory server and restoring it to new hardware is fully supported.
Bottom line is that we use Microsoft's own recommended method for preparing an AD server for backup by integrating with the VSS service to properly flag the active directlry as a backup copy. This causes it to request a new invocation ID upon restoration.
Backing up a Windows 2000 Active Directory server is also supported as long as the servers system state is properly captured with NTBackup.
Thanks.