Hello,
I had some troubles with the behavior of the RadTabControl with 2010 Q3. This has improved with 2011 Q1, or maybe I just got better at handling the Telerik controls. But it still behaves strangely when I set the TabIndex of the controls within the TabItems.
In the small example above the focus moves from the TextBox with "Text1" to that with "Text2" then to the one with "Text3" and only then to the TabItem with "Header 1", although that has a lower TabIndex and should come first. If I remove the TabIndex property from the controls within the TabItems the behavior is as expected and the focus is on the header before it is on "Text2".
In this simple example it doesn't really matter of course, the TabIndex property is not needed for correct movement within the TabItems. In all the cases I can think of right now the position of the control within the xaml code should be sufficient to ensure the right order. There may however be some situation requiring TabIndices and then they should work correctly.