While I was presenting at TechDays in Montreal last week, the question arose about clustering Active Directory.  While I have yet to find anyone who clustered AD as a service using straight MSCS or WFC, I have seen customers who virtualized Domain Controllers.

By using virtualization, you have the capability to accomplish the same goals and make AD highly available.  This can be done with VMWare, VSERVER, or Hyper-V.

Read the complete post at http://owsug.ca/blogs/brad/archive/2008/11/09/Clustering-Active-Directory.aspx