There is no difference between the way mail merge works in Silverlight 4 and Silverlight 5. With both versions, you can perform programmatic mail merge and show the resulting document in RadRichTextBox in the following way:
The prerequisites is that the mail-merge document used as template is contained in the "document" variable (it can also be the document in the editor, i.e. this.editor.Document) and that the mail merge data source of document is set.
I hope this helps.
All the best,
the Telerik team
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