richtextbox question

I thought I had a simple idea. I wanted to change my textbox into a
RichTextBox. The content of the textbox is stored in a simple sql
database with databinding, so I imagined I could store a text
representation of the flow document instead. Simple binding with the
document didn't work, so I did some searching, but nowhere in the SDK
or on the web can I find a working RichTextBox binding example. Is this
really so hard to do or am I missing something?