Mocking method with Action<T> parameter

[unit testing newbie] [c#]

Consider the following scenario:

I'm using Silverlight and calling a WCF service. Silverlight can only call WCF services asynchronously. I build a wrapper around the WCF service so that I can work with Action parameters. (makes the client code a lot cleaner).

So I have an async service that retrieves meeting rooms.

public interface IMeetingRoomService
{
    void GetRooms(Action<List<MeetingRoom>> result);
}

Turning GetRooms into List<MeetingRoom> GetRooms() is not an option.

I want to use this service in a ViewModel to set a public property called Rooms.

public class SomeViewModel
{
    private readonly IMeetingRoomService _meetingRoomService;

    public List<MeetingRoom> Rooms { get; set; }

    public SomeViewModel(IMeetingRoomService meetingRoomService)
    {
        this._meetingRoomService = meetingRoomService;
    }

    public void GetRooms()
    {
        // Code that calls the service and sets this.Rooms
        _meetingRoomService.GetRooms(result => Rooms = result);
    }
}

I want to unit test the implementation of SomeViewModel.GetRooms(). (For this question I quickly wrote the implementation but I'm actually trying to use TDD.)

How do I finish this test? I'm using NUnit and Moq.

[Test]
public void GetRooms_ShouldSetRooms()
{
    var theRooms = new List<MeetingRoom>
                       {
                           new MeetingRoom(1, "some room"),
                           new MeetingRoom(2, "some other room"),
                       };

    var meetingRoomService = new Mock<IMeetingRoomService>();

    //How do I setup meetingRoomService so that it gives theRooms in the Action??


    var viewModel = new SomeViewModel(meetingRoomService.Object);

    viewModel.GetRooms();

    Assert.AreEqual(theRooms, viewModel .Rooms);
}