XAML connections' Source and Target bindings don't work inside tab control if the diagram is not in the first TabItem.

There's a new attached property in the DiagramBehaviors - NameScopeProvider in WPF. It can be set to the RadDiagram by given element that holds the current NameScope (it's usually the root element - Window or UserControl). 

Example:
<telerik:RadDiagram x:Name="diagram"
telerik:DiagramBehaviors.NameScopeProvider="{Binding ElementName=root}"       ... />