The official version of the new 2008 R2 ADCS Migration Guide is now available at http://technet.microsoft.com/en-us/library/ee126170(WS.10).aspx.
The guide describes the necessary steps for a successful migration of both enterprise and standalone CAs to Windows Server 2008 R2 from;
- Windows Server 2003 SP2
- Windows Server 2003 R2
- Windows Server 2008
- Windows Server 2008 R2
Also included are steps for migration to Server Core.