Before you deploy SQL Server in your Windows SBS 2008 domain, be aware of the following restrictions:
The versions of SQL Server that are included in Windows SBS 2008 Premium are licensed for installation only in your Windows SBS 2008 network. You cannot install SQL Server on a server that is not in the Windows SBS 2008 domain.
You can install SQL Server on the server running Windows SBS 2008 or on the second server that you set up for your Windows SBS 2008 domain. However, for security reasons, it is recommended that you do not install the SQL Server on a domain controller. It is recommended that you install SQL Server on the second server in the Windows SBS 2008 domain.
Using SQL Server in Windows Small Business Server 2008 Premium Edition:
Windows Small Business Server 2008 Technical FAQ:
Can I install SQL Server 2008 from Windows Small Business Server 2008 Premium Edition on the first server?
Yes, this is a supported scenario. However the SQL Server 2008 management tools will not install on the same server; you must install them on another server.
Dal punto: “Installing SQL Server on a domain controller”