The Silverlight Tools don’t have multi-targeting support today, so if you want to continue building Silverlight 2 applications (for production sites), while experimenting and learning about the Silverlight 3 Beta, it isn’t that easy.
On the Silverlight Toolkit team, we leave it up to individual team members to decide what core development environment they want. By having the Silverlight 2 and Silverlight 3 SDKs checked into our source enlistment, our TFS build server can produce Silverlight 2 and Silverlight 3 binaries, and we can also msbuild the same bits on our machines, regardless of the Silverlight Tools, SDK, or runtime installed on our machines. more here