According to this post by Tim Heuer, the good news is that you can install the Silverlight 2 and 3 SDKs with Visual Studio 2010 Beta 1. You will get multi-targeted Silverlight development (i.e., either Silverlight 2 or 3) and an editable design surface.
Do not install the Silverlight 2 or 3 Tools packages, it won't work on VS 2010. And the Silverlight Navigation App template is not included with the SDK, only with the tools package.
You also need to install the Silverlight 3 Beta Developer runtime.
The bad news: .NET RIA Services will not install on VS 2010 Beta 1.