Microsoft Visual Studio 2008 SP1 Beta was released this morning. There are known incompatibilities between it and Silverlight Tools Beta 1.
To install VS08 SP1 Beta you must uninstall Silverlight Tools Beta 1 and KB949325.
Complete details can be found on the Visual Web Developer Team blog at:
Silverlight Tools Beta 2 will be release in the coming weeks and has full support for VS08 SP1 Beta.