Microsoft has released RC0 developer preparation release. This release is for preparing current Silverlight 2 projects to RTW which will be released later this year.
The release files (Visual Studio Tools, Blend, etc.) could be downloaded from here.
One important note: This release should not be used for launching Silverlight 2 applications for the general public, so if you have publicly available Silverlight application you should wait for RTW.
Most detailed information of new features, breaking changes and everything else could be found at Scott Guthrie’s blog.
Enjoy and stay tuned for RTW,