I really like how Silverlight is shaping and I hope that you enjoy your time learning and using it.
Is it an option to place the TabControl in a RadWindow?
Alternatively you may put it in a Grid panel and then use the GridSpliiter control to resize the cells of the Grid (if you do not want to put it in a window).
Here is an article about using the GridSplitter that can get you started:
Will this work in your case?
All the best,
the Telerik team