Active Directory is included as part of the /CONFIGURATION object. Ken's suggested method has a lot of validity in terms of being REALLY sure that it gets backed up
I suggest that, in the GUI, you click on Help -> Help Topics, and search for "Active Directory" (include the " marks, or you will get a lot of what is no concern for this topic) to learn more about Active Directory backup and REstore, and Disaster Recovery
If you want to see even more, I suggest that you look at theDP 5.5 Administrators Guide
pp 220-221
where it talks about Active Directory being backed up as part of the System State