The RichTextBox.Xaml Builder is a free tool that converts rich text, e.g. from a Microsoft Word document, to xml, where the xml document structure is compatible to Xaml. A xml file, produced with this tool, can be added to the Xaml property of a Silverlight 4 RichTextBox with a single line of code. You can download the tool at the Visual Studio Gallery, or as Click-Once-Install.
Additionally I crated the RichXamlViewer, a Silverlight4 UserControl, based on a RichTextBox, with some useful extra features. The RichXamlViewer has a property XamlSource, which takes the name of a xml file, created with the RichTextBox.Xaml Builder, as value. Set XamlSource and the rich text is displayed. The text inside the RichXamlViewer is zoomable at runtime using an (optional) integrated ZoomPad. Also, a context menu to copy selected text is integrated.
There is a screencast on how to easily create rich text to x(a)ml conversions with the RichTextBox.Xaml Builder and on how to use resulting xml files with a regular Silverlight 4 RichTextBox or with the RichXamlViewer. The video is in german language.