I think there is a bug in RichTextBox, which can also be reproduced in the online demo.
Steps to reproduce:
1. Open telerik editor demo - http://demos.telerik.com/silverlight/#RichTextBox/TelerikEditor
2. Select Review Tab.
3. Click Show / Hide Comments.
4. Click New Comment.
Although the comments are displayed in the editor; Show / Hide Comments button is not in checked state.