The beta version of the new 2008 R2 ADCS Migration Guide is now available at http://technet.microsoft.com/en-us/library/ee126140(WS.10).aspx.
The guide describes the necessary steps for a successful migration of enterprise or standalone CAs from Windows Server 2003 and Windows Server 2008 to Windows Server 2008 R2. Also included are steps for migration to Server Core.
This is a beta release and customer feedback would be greatly appreciated in in order to ensure the final release is all it can be. The final version is scheduled for release in March 2010. Feedback instructions are provided on the page.