We have recently improved the TabControl wrapper in regards to the TabItems so I'm happy to find it works fine for you.
The .Content property is indeed a bit useless at the time being. The framework is unable to get the object behind this type of properties in Silverlight. Improving that is in our ToDo list and we should solve that for a future version of the product.
Looking at the TabControl visual tree I think you can get the content through the ContentPresenter sub element of the TabControl I selected in the attached image. The XAML I used is as simple as this:
<!-- Create a simple TabControl in XAML. -->
Note this is a capture of the default TabControl visual tree so in case of custom template the visual tree changes.
Please let us know whether that helps or if you need further assistance.
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items