Is there a reason to make every WCF call Async?
public override User GetById(int id)
{
    return new User(service.GetUserById(id));
}

public override void GetById(int id, Action<UserGroup> callback = null)
{
    // This is a queue of all callbacks waiting for a GetById request
    if (AddToSelectbyIdQueue(id, callback))
        return;

    // Setup Async Call
    var wrapper = new AsyncPatternWrapper<UserDTO>(
        (cb, asyncState) => server.BeginGetUserById(id, cb, asyncState),
        Global.Instance.Server.EndGetUserById);

    // Hookup Callback
    wrapper.ObserveOnDispatcher().Subscribe(GetByIdCompleted);

    // Run Async Call
    wrapper.Invoke();
}

private void GetByIdCompleted(UserDTO dto)
{
    User user = new User(dto);

    // This goes through the queue of callbacks waiting
    // for this method to complete and executes them
    RunSelectIdCallbacks(user.UserId, user);
}

/// <summary>
/// Adds an item to the select queue, or a current fetch if there is one
/// </summary>
/// <param name="id">unique object identifier</param>
/// <param name="callback">callback to run</param>
/// <returns>False if it needs to be fetched, True if it is already being
/// fetched</returns>
protected virtual bool AddToSelectbyIdQueue(int id, Action<T> callback)
{
    // If the id already exists we have a fetch function already going
    if (_selectIdCallbacks.ContainsKey(id))
    {
        if(callback != null)
            _selectIdCallbacks[id].Add(callback);
        return true;
    }

    if (callback != null)
    {
        List<Action<T>> callbacks = new List<Action<T>> {callback};
        _selectIdCallbacks.Add(id, callbacks);
    }

    return false;
}

/// <summary>
/// Executes callbacks meant for that object Id and removes them from the queue
/// </summary>
/// <param name="id">unique identifier</param>
/// <param name="data">Data for the callbacks</param>
protected virtual void RunSelectIdCallbacks(int id, T data)
{
    if (_selectIdCallbacks.ContainsKey(id))
    {
        foreach (Action<T> callback in _selectIdCallbacks[id])
            callback(data);

        _selectIdCallbacks.Remove(id);
    }
}