RadRichTextBox's document model supports nested fields, however conditional fields ('IF') are not supported for now.
Keep in mind that replacing the fields directly in the document can be tricky sometimes. Instead, you could try to extend the default merge fields, as described in this forum thread
. Please find attached another sample, demonstrating custom merge field which evaluates conditionally depending on a property in the data source. The demo is for Silverlight 4, but should work for Silvelright 5 as well.
the Telerik team