Microsoft have released their white paper on how to migrate to SBS 2011 from SBS 2003. This white paper walks you through all the steps needed to take you from SBS 2003 or SBS 2003 R2 over to SBS 2011 Standard edition. This basically involves installing the new SBS 2011 server in a side by side situation with the existing server, and migrating data over to it using some manual commands on both servers, and some wizards for other things. You will have 21 days to complete the migration after which time the SBS 2003 server will shutdown every few hours. It’s a good process, a little manual, but after the initial installation of the server onsite, you can do the rest remotely.
The whitepaper is available here for download from Microsoft.com