Some customers are reporting on the Silverlight forums that they are unable to have a functional PollingDuplexHttpBinding setup, or their current polling duplex client have faulted channels. This behavior is a regression introduced in SL4 GDR1 and is visible when the following conditions are put together:

-Use of PollingDuplex
-DuplexMode is set to MultipleMessagesPerPoll
-Http stack is set to BrowserHttpWebRequest (the default stack)
-Client browser is IE8 with SL4 GDR1 or later runtime
-A poll completes after ServerPollTimeout and there is no content to consume by the client

Should you have all these conditions filled the client channel will fault. The issue will not manifest if you use any other browser.

The workaround and definitive solution would be to use the ClientHttp stack. You can do so using these instructions:

http://msdn.microsoft.com/en-us/library/system.net.browser.webrequestcreator%28v=vs.95%29.aspx

You can narrow the scope this applies to down to the specific service addresses you are accessing if you do not want to use the ClientHttp stack on all http transfers.