Directory Service Restore Mode is what you have to use in order to authoritatively restore things in active directory, like a domain controller. Or something. I've never actually used it outside of a lab until this past week, when I wanted to move where the Active Directory logs were. You can find that process here.
The shocking part came when I needed to boot into Directory Service Restore Mode (DSRM from now on, crikey). On a domain controller (and only a domain controller) you access DSRM by hitting F8 before Windows boot (just like going into Safe Mode) and then choosing DSRM from the list. I made it to safe mode, but didn't know the DSRM password. It wasn't in our password database either. Hmmmm.... and no one else knew it. It's a damn good thing that we weren't in some DR situation where we needed it!
So, please take a minute to check your DSRM password before you REALLY need it. If you need instructions on how to reset it, then look no further.