Read original post by Michael Washington at LightSwitch Help Website
The Silverlight RichTextBox is notorious for being difficult to use with MVVM (LightSwitch uses MVVM). The problem is, that unlike most controls, it does not implement a Dependency Property to bind to.
Practically any control you run across will have a Dependency Property for you to bind to. Even when you make your own Silverlight control and put it in LightSwitch, you will usually have a control like a TextBox that has a Dependency Property for you to bind to.