Seems silly but wasn’t obvious so I thought I’d share. I have a Silverlight app that is using the navigation framework. I have a punch of RichTextBoxes there with hyperlinks that wouldn’t work. The key? External hyperlinks will work when you specify Target=”_blank” on them.

<Hyperlink TargetName="_blank" NavigateUri="{Binding Uri}" ><Run Text="{Binding Author}" /></Hyperlink>