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Silverlight 2 Beta 2 has changed the concurrency model for asynchronous communications. In Silverlight 2 Beta 1, asynchronous requests always returned on the UI Thread. This was convenient, since updates to the user interface can only be done via the UI thread. As of Silverlight 2 Beta 2, asynchronous callbacks are fired in worker threads that come from a thread pool: although this potentially allows for better performance via concurrency, it increases the risk for race conditions between callbacks – more importantly, some mechanism is necessary to make code that updates the user interface run in the UI thread.