As per the following article, I have defined my service contract as
http://blogs.msdn.com/b/endpoint/archive/2010/11/13/simplified-asynchronous-programming-model-in
The support for Task-based asynchronous operations is only available in the version 4.5 of the .NET Framework Unless you're using the developer preview which was made available in mid-September, this won't work.
Here is a good answer: http://mylifeandcode.blogspot.com/2012/12/the-cause-and-solution-for.html
Seems need to re-generate proxy classes - instead of "Generate task-based operations" need to choose "Generate asynchronous operations".