MS KB 326480 has more info:
This article describes how to set up the Active Directory Migration Tool (ADMT) to perform a migration from a Windows NT 4.0-based domain to a Windows Server 2003-based domain.
You can use the ADMT to migrate users, groups, and computers from one domain to another, and to analyze the migration impact before and after the actual migration process. Make sure that you run ADMT from the primary domain controller (PDC) that is the Flexible Single Master Operation (FSMO) role holder in the target domain.
Before you upgrade a Windows NT 4.0 domain to a Windows Server 2003-based domain, the following domain and security configurations are required.
Note: This article assumes that the source domain is running Windows NT 4.0 Service Pack 4 (SP4) or later with 128-Bit encryption, and that the target domain is a Windows Server 2003-based domain in native mode. Also, the Windows Server 2003 must have 128-Bit encryption (which comes as a default setting in Windows 2003).
Note: There must be no members in this group.
Download Active Directory Migration Tool v2.0 (4.7mb)
For more information about how to use ADMT to perform a migration, see ADMT Help. Start the Active Directory Migration Tool, click Help Topics on the Help menu, click the Contents tab, and then click Active Directory Migration Tool.
Active Directory Migration Tool Overview
How to use Active Directory Migration Tool version 2 to migrate from Windows 2000 to Windows Server 2003 – 326480
HOW TO: Set Up ADMT for a Windows NT 4.0-to-Windows Server 2003 Migration – 325851