Unfortunately, we were not able to reproduce this issue using 2011 Q3 SP1 (2011.3.1220.1040) version of RadRichTextBox for Silverlight. We would appreciate it if you could provide more details on this problem for example is there something specific in the implementation of your application. It would be best if you manage to isolate the problem in a separate project which demonstrates it and send it back to us.
Thank you for your cooperation. Looking forward to your reply.
the Telerik team