Here is a quick example of this functionality. In the easiest scenario, you know the name of the html element that you want to send the value to, so when the SelectionChanged event fires on RadCoverFlow it sends that index to the control. The code for that is pretty straightforward once you get used to interacting with the DOM from Silverlight:
private void xCoverFlow_SelectionChanged(object sender, SelectionChangedEventArgs e)
HtmlDocument htmlDoc = HtmlPage.Document;
HtmlElement htmlElement = htmlDoc.GetElementById("Input");
*(the input box on the aspx page has an ID of Input).
This was built with the Q3 Beta 1 dlls, but the concept is the same going back to Silverlight 2. I've attached the project so you can check it out and use it as an example to get you started.
the Telerik team