"Preview version" means that it would be a Beta and as a Beta of the first release for WPF, there might be some features that are not working properly. We are, of course, doing our best to ship a stable version of RadRichTextBox for WPF with the full functionality of its Silverlight counterpart.
With regard to the scenario you described, you should know that RadRichTextBox is not fit to use as an HTML editor per se. It will, however, provide the functionality to format the text in the document as shown in the Silverlight demo and you will be able to export the document to HTML. How you handle the HTML from then on is up to you to decide.
As Veselin has said, the due date of the Beta is at the end of January. The official release is currently planned for the middle of March, but all these may change since there is some time until then.
the Telerik team
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