I am bound to think that the problem lies within Blend, since you do not get any exception in Visual Studio. Besides, I tried deleting and adding anew the references to the assemblies in Blend in my demo, prompting it to the right folder, but it just added references to another. That is when I got the same problem about the IsReadOnly property - with the references to the Q1 DLLs, so please make sure Blend is actually using the latest binaries.
There should not be a problem with using MVVM pattern to bind the RadRichTextBox.
If you would send me an application, illustrating your error, I might be able to assist you better.
the Telerik team
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items