Not. Async CTP has different DLLs for desktops, phones, Silverlight 4, and Silverlight 5.
Hopefully in the near future all of these platforms will support Task (and async) initially, and then portable class libraries will be able to use async.
Update: Microsoft.Bcl.Async can be installed for portable libraries designed for .NET 4.0 / 4.5, applications for the Windows Store, Silverlight 4/5, and Windows Phone 7.5 / 8.0
Stephen cleary
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