The first public CTP of Katmai will be available for download from http://connect.microsoft.com/sqlserver.  

The name for the next release of SQL Server™: Microsoft SQL Server 2008, formerly code-named SQL Server “Katmai,” and the delivery of the first SQL Server 2008 Community Technology Preview (CTP), now available for customer download at http://www.microsoft.com/sql/prodinfo/futureversion/default.mspx

 Be sure to check out the live web cast of Dave Campbell's Katmai Overview presentation at Tech Ed in Orlando.