Some folks have already started trying out the latest VS2010 release, and have been receiving installation failures for the Silverlight 3 SDK. This is a known issue, and fairly easy to work around.
The problem is caused by having the Silverlight 3 RTW version of the SDK installed (3.0.40624.0). VS2010 tries to install the Silverlight 3 GDR2 version (3.0.40818.0), which doesn’t automatically upgrade in place. The GDR version fails to install, giving this error.
This does not completely break Silverlight development scenarios, as some people are already reporting. However, there may be some minor issues that come up if you’re using the Silverlight 3 RTW SDK. To fix this issue, you can uninstall the Silverlight 3 SDK, and install the latest version from http://go2.microsoft.com/fwlink/?LinkID=157102.
We are working on providing a smooth experience for the VS2010 RTM release.