Exchange DR planning ? realizing the importance of backing up the Directory

Virtualization of one or more DC's ? i.e. running those DC's as virtual servers on a supported virtualization platform, can provide the benefits when it comes to recovery, including being able to take a DC offline and copy it into a lab for Disaster Recovery (DR) scenario testing.

Third party AD recovery tools add huge value to this space ? a number of these tools allow a backup to take place several times a day  per domain/DC or site, the schedule here depends on how often your forest updates. A word of caution here is that recovery tools should augment your AD backup strategy; they shouldn't be your backup strategy.

Lag sites are well documented, so is the virtualization of Domain controllers, so I won't cover those here. The other factors I mentioned will come into play based on lab experience, talking to the business, object and attribute owners (Exchange Administrator, Unix schema extensions, group owners, etc), since you are looking to preserve the objects, linked relationships (groups, managers, exchange), plus the services AD and Exchange provide LDAP, GC's,  lookups, authentication, mail routing etc.

Summary
Disaster recovery is a huge subject for most companies, it helps to have a multi disciplined approach and a follow on plan for the things affected by a directory outage.  Research (Forrester), shows that 35% of all backup tapes do not contain what people expect. An eyebrow raising statistic!  Best practices would suggest that while you are testing your DR plan you perform a number of test restores to ascertain the reliability of your backup set.

 Related Links
Exchange Server 2003 Technical Documentation Library
http://www.microsoft.com/technet/prodtechnol/exchange/2003/library/default.mspx

Active Directory Disaster Recovery for Branch Office Environments
http://www.microsoft.com/technet/archive/windows2000serv/technologies/activedirectory/deploy/adguide/addeploy/addch10.mspx?mfr=true

Windows Server 2003 Active Directory Diagnostics, Troubleshooting, and Recovery
http://www.microsoft.com/technet/community/events/windows2003srv/tnt1-80.mspx