Laurent Bugnion just published MVVM Light Toolkit V3 Alpha 3 which support WPF 4 and Silverlight 4!
I published, yesterday a short post in French on my Tech Head Brothers portal about Silverlight 4 Beta, Drop Target et MVVM which shows the usage of this new version of EventToCommand combined with Silverlight 4 Drop Target.
Here is the post of Laurent
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!