Because of un-addressable client in SL, we introduced Polling Duplex channel to support callback in WCF Silverlight. It started as a channel in Silverlight2. We added service model support in Silverlight3 and MultipeMode, aka chunking in Silverlight4. See MSDN for more information about how to use it.
Recently some customer reported that session state object in httpcontext.current is null in server side while using PollingDuplex binding ( ...
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