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.
At Innoveo Solutions we are using .NET and WPF for our Innoveo Skye® Editor application. Skye® Editor is a distribution channel editor targeting business people letting them edit and configure their insurance products.
From the beginning we have adopted the Model-View-ViewModel architecture. Having our solution growing we were facing the issue of having our ViewModels dependency growing too. Some ViewModel became too much dependent of others. This was obvious in our unit tests whose complexity to setup were growing too. It was time to find a solution to decouple the ViewModels.