For general background of SBS 2011 Premium Add-on, please see
SBS 2011 Premium Add-on Datasheet.
SBS 2011 Licensing FAQ

1. How can I get a trial version of SBS 2011 Premium Add on?
We don’t have a specific trial version for SBS 2011 Premium Add-on. However, you can get the trial version of individual products from following links: 
Windows Server 2008 R2  
SQL Server 2008 R2.

2. What specific products or licenses do I get from purchasing SBS 2011 Premium Add-on?
Please refer to the
SBS 2011 Premium Add-on Datasheet.

3. What virtualization rights do I have with the SBS 201 Premium Add-on?

With the SBS 2011 Premium Add-on, you can run one instance of Windows Server 2008 R2 Standard technologies in a physical or virtual operating system environment (OSE).
If you run it in a virtual OSE, you are allowed to run an additional instance for the physical OSE in order to run hardware virtualization software, provide hardware virtualization services, or run software to manage and service virtual OSEs on the licensed server.

4. Can I use the SBS 2011 Premium Add-on to virtualize SBS 2011 Standard or Essentials?
Yes, both editions can be virtualized.

5. Can I install and use Windows Server Remote Desktop Server (RDS) Session Host on my Premium Add-on server?
Yes.  Note that you would need to acquire RDS CALs.

6. I have licensed SharePoint 2010 Server, SQL server and Windows 2008 R2 server with my SBS server. After migration to SBS 2011 Essentials, do I need to buy Premium Add-in to keep these server licensed?
No, you don’t need buy new licenses.

7. I have SQL trial version installed on the SBS 2011 Essentials server. After I buy the SBS 2011 Premium Add-on, how could I replace my SQL trial with SQL 2008 R2 for Small business?
Please see SQL product guidance on whether there is upgrade path from trial to licensed version of SQL. There is no additional limitation on SBS side.

8. From license point of view, Can I install the SQL Server 2008 R2 included in the SBS 2011 Premium Add-on on SBS 2008 or 2011 Standard first, and then migrate to SBS 2011 Essentials?
You can only run the SQL Server component of the PAO on one server at a time.  If you install in on an SBS 2008 server and later want to migrate to SBS 2011, then you could uninstall it from the source server and install it on the destination server as part of the migration process.

9. With the Premium Add on, Can I install SQL server 2008 R2 on SBS 2011 Essentials, instead of on the separate Windows Server 2008 R2 server?

10. What’s the installation guidance for Premium Add-on?
There is no SBS specific step for the Premium Add-on installation. Please follow the general installation guidance for Windows Server Standard 2008 R2 and SQL Server 2008 R2.

11. Can SBS 2011 Premium Add-on server join SBS 2011 Essentials, and work with RWA and RDS?
You can join the Windows 2008 R2 server (included in SBS 2011 Premium Add-on) to SBS 2011 Essentials domain via Windows Native Domain Join process.
However you will not be able to install SBS 2011 Essentials Client Connect on the Windows 2008 R2 server, the SBS 2011 Essentials client features, including RWA, Client Backup or Health Alerts will not be supported on the Windows 2008 R2 server.