"Microsoft .NET Framework 3.5 service pack 1 is a cumulative update that contains many new features building incrementally upon .NET Framework 2.0, 3.0, 3.5, and includes .NET Framework 2.0 service pack 2 and .NET Framework 3.0 service pack 2 cumulative updates.

IMPORTANT This beta release is for early testing and feedback. As a beta release, we recommend you only install the service pack on test computers. Prior to installation, you should carefully review the included readme file to be aware of any known issues with this release. For example, this service pack has some known compatibility issues with previous releases, including:

Expression Blend (all versions)
Silverlight 2 Beta 1 SDK
Silverlight Tools Beta 1 for Visual Studio 2008

A new release of Expression Blend 2.5 March Preview that is compatible with this service pack is available."