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SQL Server 2008 has been released a few days ago; you can download it from http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=66ab3dbb-bf3e-4f46-9559-ccc6a4f9dc19. Please read the release notes (http://go.microsoft.com/fwlink/?LinkId=147331).
this is the list of SP1 fixes (http://support.microsoft.com/kb/968369/).
Aditionally, you should pay attention that some fixes included in SQL Server 2008 CU4, are not included in SP1, (http://blogs.msdn.com:80/psssql/archive/2009/04/09/sql-server-2008-sp1-and-cumulative-updates-explained.aspx).