To add a 2008 domain controller to a 2003 domain:

Use 2008 R2 media disk (on the 2003 server) and go to the support folder. Run adprerp32.exe /forestprep.  After 15 minutes or so (for a small domain) run adprep32.exe /domainprep

If you have group policies you need to run adprep32.exe /gpprep

Now you can run dcpromo on the 2008 server.

Enjoy the lovely DNS warnings!