Currently, saving the RadTreeView or other telerik Silverlight control`s state is not implemented out of the box. However, we have it in our plans - creating a persistence framework is scheduled for Q2 2011.
Thank you for your interest in this feature.
the Telerik team
Registration for Q1 2011 What’s New Webinar Week is now open. Mark your calendar for the week starting March 21st and book your seat
for a walk through all the exciting stuff we ship with the new release!