Async File IO is not async?

A very simple test program which writes out 512 MB:

using System;
using System.IO;
using System.Threading;

namespace AsyncIOTest
{
    class Program
    {
        static void Main()
        {
            FileStream file = File.Create("out.test", 0x1000, FileOptions.Asynchronous);

            ManualResetEvent evt = new ManualResetEvent(false);

            Byte[] buffer = new Byte[512 * 1024 * 1024];
            file.BeginWrite(buffer, 0, buffer.Length, Program.OnWriteComplete, evt);

            evt.WaitOne();
        }

        static void OnWriteComplete(IAsyncResult ar)
        {
            ManualResetEvent evt = (ManualResetEvent)ar.AsyncState;

            evt.Set();
        }
    }
}

On my machine BeginWrite takes about 12 sec followed by immediate call to OnWriteComplete. A quick examination with Reflector reveals that BeginWrite and BeginRead do not satisfy most (if not all) of the requirements of a "true" async IO outlined here: http://support.microsoft.com/kb/156932.

A bug?

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