This was released (RTM) on 6th August 2008

This is repeated in the wikipedia SQL Server 2008 article:

http://en.wikipedia.org/wiki/SQL_Server_2008#SQL_Server_2008

SQL Server Home Page
http://msdn.microsoft.com/en-us/sqlserver/default.aspx

At a Microsoft presentation I attended in February I was told that SQL Server releases would be more frequent - 1.5 - 3 years. There won't be any more long gaps as there was between SQL Server 2000 and 2005.