SBS 2008 Premium includes everything from SBS 2008 Standard plus: SQL 2008 Standard for Small Business and an extra Server 2008 Standard license for running a 2nd server (e.g. for Terminal Services)
Source: http://www.microsoft.com/sbs/en/us/compare-features.aspx
Windows Small Business Server (SBS) 2008 combines several server technologies for an all-in-one small business network suite.
SBS 2008 comes in two editions: Standard and Premium.
The diagram and chart below detail the various product technologies included in each edition.
1 120 day trial included in the product.
2
Office Live Small Business is available in the following languages
(countries): English (Canada, U.K., U.S.); French (Canada, France);
German (Germany); and Japanese (Japan).
3
Microsoft Forefront Security for Exchange Server is available in the
following languages: English, French, German, Italian, Japanese, Korean,
Chinese (Simplified), Chinese (Traditional), Portuguese (Brazil),
Spanish, and Russian.
4Windows Server 2003 R2 Standard is available as an alternative version in place of Windows Server 2008 Standard.
5
Through June 30, 2010, customers using applications that are not yet
certified for use on SQL Server 2008 Standard, a copy of SQL Server 2005
Standard will also be included in SBS 2008 Premium. SQL Server 2008 (or
2005) Standard can be installed on either server: If you install SQL on
the first server then you must install the management tools on another
machine.
For information on Windows Live OneCare for Server, visit the OneCare Update page.
To learn more about product licensing and get additional details on Client Access Licenses, visit the Licensing page.