Vista - Asynchronous WCF Question

Hi
I am using WCF Services exposed as REST URIs to be consumed by generic AJAX
clients. All of my services need to be Async in nature.
We have a typical n-layer architecture for our project wherein the Service
Layer --> Business Layer and
Business Layer --> Data Access Layer
Data Access Layer --> LINQ --> DB

The heaviest part of the processing will be fetching and sending updates to
the database. As such this part needs to be made asynchronous.
Earlier, we had thought that the service layer would simply accept the
calls. It would then use ThreadPool.QueueUserWorkItem to spawn a secondary
thread and move the calls to the Business Layer onwards to this secondary
thread. For e.g.

public IAsyncResult BeginGetData(params)
{
//Create an AsyncResult
ThreadPool.QueueUserWorkItem(SomeCallback, AsyncResult)
//Return AsyncResult
}

private void SomeCallback(object state)
{
//Call BUSINESS Layer which calls Data Access Layer and so on.
}

public string GetData(AsyncResult)
{

}
But this defeats the purpose of Async operations since we end up using
threads from the same thread pool as ASP.Net for the WCF services. A load
test confirmed this too. The performance was worse than a synchronous WCF
service call. An article I read on Async HTTP handlers mentioned the same. Do
I need to split my Business Layer and Data Access Layer methods into Begin
and End pairs too? Or is there a better way.

Kindly let me know, what is the preferred way of handling this as I am sure
that this would be a problem faced commonly.

Thanks in advance
Vikas Manghani