First, you cannot copy/paste formatted text in Silverlight - the clipboard is limited to access plain text only.
Anyway, if you still need to add formatting options for the "Description" field in EditAppointmentDialog, you can use RadRichTextBox control. In combination with HtmlDataProvider, it will return the entered text in HTML format, which you should be able to save in the database without a problem.
In order to display the Description, you should customize the AppointmentItemContentTemplate and add again RadRichTextBox in order to render the HTML.
I have attached an example showing the described approach, please download it and give it a try.
the Telerik team