I can do it like this:
RadTabItem tab =
tab.Content = item;
tab.Header = item;
tab.ContentTemplate = (DataTemplate)
tab.HeaderTemplate = (DataTemplate)
I have to do this in codebehind and set Content and Header to the data. Know this, because I had the same problem at the RadBook.
Don't really understand why DataContext is not working in this two cases and I have to set the Conten with a DataObject.
Isn't Content almost made for controls and DataContext for the data? Anyway, I got it.