Async WCF self hosted service

My objective is to implement an asynchronous self hosted WCF service which will run all requests in a single thread and make full use of the new C# 5 async features.

My server will be a Console app, in which I will setup a SingleThreadSynchronizationContext, as specified here, create and open a ServiceHost and then run the SynchronizationContext, so all the WCF requests are handled in the same thread.

The problem is that, though the server was able to successfully handle all requests in the same thread, async operations are blocking the execution and being serialized, instead of being interlaced.

I prepared a simplified sample that reproduces the issue.

Here is my service contract (the same for server and client):

[ServiceContract]
public interface IMessageService
{
    [OperationContract]
    Task<bool> Post(String message);
}

The service implementation is the following (it is a bit simplified, but the final implementation may access databases or even call other services in asynchronous fashion):

public class MessageService : IMessageService
{
    public async Task<bool> Post(string message)
    {
        Console.WriteLine(string.Format("[Thread {0} start] {1}", Thread.CurrentThread.ManagedThreadId, message));

        await Task.Delay(5000);

        Console.WriteLine(string.Format("[Thread {0} end] {1}", Thread.CurrentThread.ManagedThreadId, message));

        return true;
    }
}

The service is hosted in a Console application:

static void Main(string[] args)
{
    var syncCtx = new SingleThreadSynchronizationContext();
    SynchronizationContext.SetSynchronizationContext(s