Microsoft has just released first beta of upcoming Silverlight 5.

Microsoft Silverlight 5 Beta Tools for Visual Studio 2010 Service Pack 1 is an Add-on and pre-requisite files for Visual Studio 2010 Service Pack 1 to develop Silverlight 5 Beta and Microsoft WCF RIA Services V1.0 SP2 Preview (April 2011) applications.

Download: Silverlight 5 Tools Beta

Microsoft® Silverlight™ 5 Software Development Kit Beta

Silverlight 5 Beta Documentation

Getting Started with Silverlight 5 Beta