Save/Load layout feature of Docking control persists the layout, but it must be used explicitly. Here you can find helpful information about this feature - http://www.telerik.com/help/silverlight/raddocking-features-save-load-layout.html
Also, you could use the following example as a start point when implementing Docking with PRISM - http://blogs.telerik.com/xamlteam/posts/13-12-11/using-raddocking-with-prism-in-wpf-silverlight
i hope this information helps.
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