Update: Fixed some links, corrected some text.
With the Silverlight Toolkit they are taking a new approach to shipping new controls in an effort to move swiftly and deliver the controls to people at the right time. Their previous approach was to ship the Toolkit when every component was ready, and completely fleshed out.
Now they will be shipping the Toolkit with controls that might have different levels of quality (and they are clearly flagged in the documentation). Shawn explains the new "Quality Bands" model that they are using in his post.
The source code for the Toolkit and the Controls is great to learn how to use Silverlight and it is great for people that need to tweak them for their own applications. When it comes to these controls, you no longer need Microsoft to make small changes for you or the small bug fixes that impact your application.
Themes: An interesting control container in Silverlight is the theming control. You wrap your code around this, and it will let you skin your control with XAML and define the animations and interactions with XAML and the Visual State Manager:
Some of these themes reminded me of the Gtk+ themes from 1998. Back in the days of Enlightenment and the "Cheese Pixmap" theme were hot. Mehdi explains how the themes work and Jafar explains the ImplicitStyleManager, the foundation for themes.