I was doing some research before a domain migration at a customer site when I found this great forum post over at the techarena forum. http://forums.techarena.in/server-migration/1010582.htm

The general steps to migrate from Windows Server 2000 to Windows Server 2008 is almost the same as the steps to migrate from Windows Server 2003 to Windows Server 2008.

Scenario:
Old server: Windows Server 2000 box
New server: Windows Server 2008 box

1. Verify that Windows 2000 SP4 have been installed on the old server and Windows Server 2008 SP1 have been installed on the new server.  Note: the domain functional level should be in Windows 2000 native mode for the Windows 2000 domain
2. Upgrade the Windows 2000 forest schema by running “adprep /forestprep” command on old server. Note: you can copy the adprep folder from the Windows Server 2008 installation disc to the old server to run “adprep /forestprep”. The location is \source\adprep on the installation disc.
3. Upgrade the Windows 2000 domain schema by running “adprep /domainprep” command on old server
4. Verify the new server’s TCP/IP configuration has been pointed to existing DNS server.
5. Add new server to existing Windows 2000 domain as a member server.
6. Run dcpromo on new server to promote it as an additional domain controller in existing Windows 2000 domain, afterwards you may verify the installation of Active Directory.
7. Enable Global Catalog on new server and manually Check Replication Topology and afterwards manually trigger replication to synchronize Active Directory database between 2 replica.
8. Disable Global Catalog on old server.
9. Use NTDSUTIL utility to transfer all the 5 FSMO roles from old server to new server. You’d better transfer FSMO roles via GUI method instead of using NTDSUTIL.
10. Install DNS component on new server and configure it as a new DNS Server(Active Directory Integrated-Zone is preferred). Note: all the DNS configuration need to be transferred from the existing DNS Server.
11. Migrate DHCP service from Windows Server 2000 to Windows Server 2008. Migrate File Sharing service. You can also use robocopy.exe from the Windows Server Resource kit to copy the files and maintain NTFS permissions.
    For the shares, the shares definitions and permissions are stored in the following registry key on the server: “HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\lanmanserver\Shares”. If you export out the whole key and import in the new server, and if you have the files and folders in the same path as in the old server, then you will retain all you shares and share permissions.
12. It’s better to make the old DC offline for several days and check whether everything works normally with the new server online. If so, you may let the old DC online and run DCPROMO to demote it.

For more information, please refer to:

• How to move a DHCP database from a computer that is running Windows NT Server 4.0, Windows 2000, or Windows Server 2003 to a computer that is running Windows Server 2003 (This should work on Windows Server 2008) http://support.microsoft.com/default.aspx/kb/325473 
• How To Install and Configure DNS Server in Windows Server 2003 http://support.microsoft.com/kb/814591/EN-US/ 
• How To Convert DNS Primary Server to Active Directory Integrated http://support.microsoft.com/kb/816101