Microsoft is working on ADMT 3.2, which can be installed on Windows Server 2008 R2 servers for migrations. But there's no estimated date yet. In the meantime, a ADMT 3.1 comes with supported scenarios for using it to migrate to R2 domains. Read more about this:

• KB976659 - Known issues that may occur when you use ADMT 3.1 to migrate to a domain that contains Windows Server 2008 R2 domain controllers.
• KB974625 - You cannot uninstall ADMT 3.1 after you perform an in-place upgrade to Windows Server 2008 R2.