We recently published Capacity Planning for Active Directory Domain Services. This is a a detailed and comprehensive set of guidelines to help optimize computing resources for DCs, including considerations for virtual environments. It's by Ken Brumfield, a Senior field engineer here at Microsoft, based on his extensive experience and research. This has been a frequently requested document over the years and the Active Directory documentation is grateful to Ken for sharing his expertise and time to fill this need.
The wiki version is published to http://social.technet.microsoft.com/wiki/contents/articles/14355.capacity-planning-for-active-directory-domain-services-en-us.aspx.
Justin [Active Directory Documentation Team]