Vista - transferMode="Streamed" does not stream within IIS

I have a WCF service hosted within IIS that uses the following custom
binding:

<customBinding>
<binding name="Streaming">
<httpTransport maxReceivedMessageSize="67108864"
transferMode="Streamed" />
</binding>
</customBinding>

My service operation is defined as:
[OperationContract]
Stream GetData(string sql);

When hosted using a ServiceHost (console applicaiton), the stream works
as expected. The client immediatley returns from GetData and can begin
accessing the stream.

However, when I host the same service within IIS (5.1 and 6), the
client does not return from the GetData call until the stream is
completed (closed).

It appears that ASPNET is buffering the entire contents of the stream
prior to responding to the client. I tried changing the bufferSize on
httpTransport, but that didn't work.

Any suggestions?