Q3 2012 SP1 has a bug at the way we check if the async feature is available. You can workaround it if you create an empty Silverlight class library called AsyncCtpLibrary and then add an assembly reference to it from the Silverlight project. That way you should have the async feature enabled. You project needs to have an assembly reference to AsyncCtpLibrary.
Please note that the class library should not be part of the solution as it needs to be an assembly reference, not a project reference.
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