You can find a list of version, build date informations for Microsoft SQL Server 2008 (Katmai) CTP's, releases, etc builds here.
Here is CTP4 (June CTP) for SQL Server 2008 Developer Edition installed on a Windows Server 2008 Server Beta 1
Microsoft SQL Server code name "Katmai" - 10.0.1019.17 (Intel X86)
May 24 2007 15:26:55
Copyright (c) 1988-2007 Microsoft Corporation
Developer Edition on Windows NT 6.0 (Build 6001: Service Pack 1, v.126)
Here is CTP5 (November CTP) for SQL Server 2008 Developer Edition installed on a Windows XP SP2
Microsoft SQL Server code name "Katmai" (CTP) - 10.0.1075.23 (Intel X86)
Nov 8 2007 14:16:26
Copyright (c) 1988-2007 Microsoft Corporation
Developer Edition on Windows NT 5.1 (Build 2600: Service Pack 2)
Here is CTP5 (November CTP) for SQL Server 2008 Express Edition installed on a Windows XP SP2
Microsoft SQL Server code name "Katmai" (CTP) - 10.0.1075.23 (Intel X86)
Nov 8 2007 14:16:26
Copyright (c) 1988-2007 Microsoft Corporation
Express Edition on Windows NT 5.1 (Build 2600: Service Pack 2)