The problem you are facing is due to a problem in the official SL3 beta release. We have tested the RadDocking with a newer build of the SL3 runtime which was provided by Microsoft and the issue appeared to be fixed there. So you will have a properly working version of the RadDocking with the official SL3 version which is due soon enough. We are aiming at releasing our official RadControls for Silverlight 3 shortly after MS launches the official Silverlight 3.
the Telerik team