I just published the latest alpha version of the MVVM Light Toolkit. I will post more about the new features in this alpha version, but the most exciting is probably that with this release, the MVVM Light Toolkit works in Windows Presentation Foundation 4 and in Silverlight 4 (in Visual Studio 2010).
There is no automatic installer for this version yet, but I wrote a page describing how to install manually (it is as easy as unzipping a few files). All the features are also available for Visual Studio 2008 as usual.
The source code was also updated on the Codeplex website.
More documentation will follow soon so stay tuned. In the mean time, have fun with MVVM in WPF4/SL4!