Today the Small Business Server User Assistance team has finished writing up the technical documentation for the Migration from both Windows SBS 2003 to Windows SBS 2008, as well as Windows SBS 2008 to Windows SBS 2008. The documentation is available on the Windows SBS 2003 Technical Documentation Library.
Direct links if you're interested are here:
But wait, there is more, what if you're on site and you don't have access to the web while doing the migration. The UA team has you covered there with the offline, complete migration help CHM File.
Get familiar, use the Public Beta to practice.