Microsoft has released the CTP Edition (Community Technology Preview) of MS SQL Server 2008 Service Pack 1.
A public available free download of MS SQL2008 SP1 can be found at Microsoft Downloads - SQL Server 2008 Service Pack 1 - CTP
The SP1 downloads are about 260 - 320 MB in sizes.
SQLServer2008SP1-KB959337-IA64-ENU.exe 313.3 MB
SQLServer2008SP1-KB959337-x64-ENU.exe 324.0 MB
SQLServer2008SP1-KB959337-x86-ENU.exe 262.8 MB
And the MS SQLServer 2008 Express Edition SP1 CTP are available at Microsoft Downloads - SQL Server 2008 Express Edition Service Pack 1 - CTP
and are in sizes about 80-105 MB.
The installation is smooth and caused no problem or faced with no bug during installation.
After all my SQLServer 2008 instance build number is as Microsoft SQL Server 2008 (SP1) - 10.0.2520.0