Simple Mvvm Toolkit v2 is more robust and easier to use. The requirement to unregister when using the MessageBus has been removed because it uses a leak-proof eventing model. You now send and receive messages by calling convenient methods on ViewModelBase. The ViewModelLocator now creates ViewModels on-demand instead of storing them in a dictionary, so that the lifetime of the ViewModel is controlled by the View. The only breaking change is that the DataContext in the View is bound directly to the locator property instead of using indexer syntax with square brackets. There is also better support for testing with the Silverlight Unit Testing framework, which comes with the Silverlight Toolkit.
The main addition to functionality and ease-of-use is the inclusion of Visual Studio project templates for Silverlight, Windows Phone and WPF, including a template that uses WCF RIA Services. Each project template provides a working MVVM application with all the classes you need to get started. There are additional samples as well (Main Parts 2-3).
Most importantly, instead of manually installing the toolkit, the download now consists of an installer that automatically copies templates, code snippets, binaries, samples and source code to a SimpleMvvmToolkit directory under Program Files (Program Files (x86) on 64-bit machines).