Just a note for those of you who may be starting with CM 2012. It is not necessary to extend the AD schema with CM 2012 if you extended it for SCCM 2007.

See here:

http://technet.microsoft.com/en-us/library/gg699359.aspx#WhatsNew_Sites_and_Hierarchies

Note
The Active Directory schema extensions for System Center 2012 Configuration Manager are unchanged from those used by Configuration Manager 2007. If you extended the schema for Configuration Manager 2007, you do not need to extend the schema again for System Center 2012 Configuration Manager.