Microsoft uploaded a public beta of Silverlight 5, you can grab it over here.
Microsoft Silverlight is a powerful tool for creating and delivering rich Internet applications and media experiences on the Web. Silverlight 5 builds on the foundation of Silverlight 4 for building business applications and premium media experiences. Among other capabilities, the Silverlight 5 beta highlights dramatic video quality and performance improvements, and features that improve developer productivity.
The final Silverlight 5 release will be available in 2011.