[Full Disclosure: I work for Telerik.]
Rich text editing support in Silverlight is a challenge at the moment due to limits in the Silverlight plug-in. The plug-in does not yet expose a good way to work with rich text, and it also lacks any support for printing. These are features that the core SL plug-in team are working on, so I expect that by late this year, early next we'll have a plug-in much more capable of supporting rich text editing.
In the mean time, Telerik does have a basic rich text editor for manipulating HTML. We rely on the Silverlight DOM integration to provide rich HTML editing support in Silverlight, but it is a good itermediate solution, espeically if your text is HTML formatted. Details: RadEditor for Silverlight
Longer term, you can expect to see a complete rich text editor from Telerik that will leverage Silvelright's native editing APIs (when they are eventually added to the plug-in). We think these enhancements are around the corner, so we're waiting for that instead of jumping the gun with a proprietary rich text editing layer that will be quickly rendered obsolete by the framework.
Hope that helps clarify the Silverlight rich text editing / printing picture today.
You can check out the requests for RTF support in Silverlight 4 in this Silverlight.net forum thread:Silverlight 4 Wishlist