In this article, Josh Twist discusses a MEF-driven ViewModel navigation scheme with animation on the navigation.
One of my favourite features in WPF is the ability to have it automagically wire up a databound object (often a ViewModel) to a particular DataTemplate. A lot of people really miss this feature when using Silverlight as it's still missing from the latest version (Silverlight 4 at the time of writing).
Folk have blogged a number of bespoke solutions, including one by my colleague Rob Garfoot: Flexible Data Template Support in Silverlight.