The following TechNet article – “Migrate to Windows Small Business Server 2008 from Windows Small Business Server 2003” – covers the process of installing Windows Small Business Server 2008 and migrating your existing Windows Small Business Server 2003 based domain:

http://technet.microsoft.com/en-us/library/cc546034.aspx

Although the process is fairly straight forward and simple, we have found it useful to simulate the process using a virtualized environment created from the SBS 2003 machine using Acronis True Image Virtual Edition and VMWare.