I just downloaded the latest production version and upgraded my project and have run into manu issues within 2 min of running my application. On the tabcontrol, the selected item is behaving differently. If you run your databinding example for the tab control and use the "x" to close the tabs you'll notice there is no selected item afterwards. If you run the 2010 demo it works fine. I am using Prism and MVVM and have a solution that works with the 2010 Q3 release but now does not work now that I have upraded (similar to how I explained it worked in your demo).