Hello Kirk Quinbar,
RadRichTextBox uses DataProviders to utilize Binding, so that it can be more flexible and support different formats - XAML, HTML, plain text, DOCX.
As it comes to your scenario, I have tested it and it seemed to work fine. I may be doing something differently, so I am sending you my testing demo. Look through it and if it doesn't help you address the problem, feel free to get back to us with a project of yours.
the Telerik team
Browse the videos here>>
to help you get started with RadControls for Silverlight