The long awaited 2008 R2 version of ADMT has been released to the web. You can download it here:
A good read, if you’re looking at using this tool is:
Active Directory Migration Guide
Active Directory Migration Tool (ADMT) Guide: Migrating and Restructuring Active Directory Domains
However for complex migrations/transitions/whatever I prefer the Quest Migration Manager for Active directory.
Here is some info from the ADMT download page:
The Active Directory Migration Tool version 3.2 (ADMT v3.2) provides an integrated toolset to facilitate migration and restructuring tasks in an Active Directory Domain Services infrastructure.
The Active Directory Migration Tool version 3.2 (ADMT v3.2) simplifies the process of migrating objects and restructuring tasks in an Active Directory® Domain Service (AD DS) environment. You can use ADMT v3.2 to migrate users, groups, service accounts, and computers between AD DS domains in different forests (inter-forest migration) or between AD DS domains in the same forest (intra-forest migration). ADMT can also perform security translation (to migrate local user profiles) when performing inter-forest migrations.
- Supported Operating Systems: Windows Server 2008 R2
- ADMT can be installed on any computer capable of running the Windows Server 2008 R2 operating system, unless they are Read-Only domain controllers or in a Server Core configuration.
- Target domain: The target domain must be running Windows Server 2003, Windows Server 2008, or Windows Server 2008 R2
- Source domain: The source domain must be running Windows Server 2003, Windows Server 2008, or Windows Server 2008 R2
- The ADMT agent, installed by ADMT on computers in the source domains, can operate on computers running Windows XP, Windows Server 2003, Windows Vista, Windows Server 2008, Windows 7, and Windows Server 2008 R2.
- PES v3.1 is a separate download also available on the Microsoft Download Center. See the Related Downloads section below.
- ADMT v3.2 is the last version of the tool which will support migration operations involving Windows Server 2003, Windows Server 2008, or Windows Server 2008 R2 source domains, target domains, or domain controllers.
- To obtain customer support if you are performing migration operations involving NT 4.0 (with SP4 or higher) or Windows 2000 Server source domains, or domain controllers, please contact your Microsoft Services representative or visit http://www.microsoft.com/microsoftservices.