text: Communicating Between Silverlight and ASP.NET
  If your website includes ASP.NET code and Silverlight content, you may want to pass some information from one side to another. There are several ways that you can allow this sort of interaction. Here are two simple approaches:
  - When you redirect the user from an ASP.NET web page to a Silverlight entry page, you can supply startup information in a cookie or a query string. The Silverlight code can access this information using the classes in the System.Windows.Browser namespace.
  - Similarly, your Silverlight application can use the query string or set a cookie before navigating to an ASP.NET web form by calling HtmlPage.Navigate(). If the Silverlight control is on an ASP.NET web form, it can call HtmlPage.Submit() to trigger a postback.
  Both of these techniques allow you to send information as you switch from the client side to the server side, and vice versa. For example, if your Silverlight control calls HtmlPage.Submit(), the entire page is posted back, the Silverlight application ends, and the ASP.NET objects are created. If you want the user to perform another action with your Silverlight application, you need to return a new page with the Silverlight content. Then, the Silverlight control needs to be created and initialized all over again.
  Another option is to allow a long-running Silverlight application to trigger some server-side code without actually posting back the page. That way, the application continues running. The easiest way to do this is to have your Silverlight application call an ASP.NET web service, just as you would with an ASP.NET AJAX page. The development model is quite convenient -- when you add a web reference to your web service in the Silverlight project, Visual Studio generates the proxy class you need to call the web service. You simply need to instantiate the proxy class and call its methods, just as you would in a full-fledged .NET application that calls a web service.