Primers for building a highly available Active Directory environment
Notes from the field on things to consider with regards to maintaining Active Directory:
Hardware Diversity - this includes virtualization and SAN's. Read the official Microsoft notes on virtualization recommendations in the Technet article and KB below.
DC's are designed to be redundant and distributed - try to avoid putting all DC's in the same domain on the same SAN or virtualization host.
Physical Isolation - try to maintain at least two DC´s in each domain and physically separate the two, preferably to separate locations, buildings or rooms. Buildings burn down, rooms get flooded, etc.
Making sure any backups are not stored on the same SAN that stores the servers being backed up would also be advisable.
Multiple backups - an extra Windows Backup set up on the DC to periodically back itself up can complement any other backup solution you have in place. Cheap redundancy is always a good thing.
Fire Drills - test the backup can be restored to a bare-bones installation (See KB249694). Prepare a worst case DR plan and define who does what during the scenario and roleplay through it (sitting around in a room and feeling stupid in a role-playing scenario is better than feeling the same while under fire).
Prioritize - If you have multiple DC's in a domain it may be quicker to bring up just one and then dcpromo more into the domain rather than spending time on restoring multiple DC's in a Disaster Recovery scenario. It all depends on the severity of the situation...
Further reading:
Things to consider when you host Active Directory domain controllers in virtual hosting environments
http://support.microsoft.com/kb/888794
Running Domain Controllers in Hyper-V
http://technet.microsoft.com/en-us/library/virtual_active_directory_domain_controller_virtualization_hyperv(WS.10).aspx
How to move a Windows installation to different hardware
http://support.microsoft.com/kb/249694
Appendix A: Forest Recovery Procedures
http://technet.microsoft.com/en-us/library/cc781218(WS.10).aspx
The Golden Rules of Resource Management
http://blogs.technet.com/b/instan/archive/2009/06/22/the-golden-rules-of-user-resource-management.aspx