ComboBox SelectedValue doesn't show

I have a strange problem in my WinRT/C# XAML Metro app, using the Windows 8 Release Preview (latest patches installed). I'm using a ComboBox, whose values ItemsSource and SelectedValue are bound to properties in a ViewModel:

<ComboBox SelectedValue="{Binding MySelectedValue, Mode=TwoWay}"
          ItemsSource="{Binding MyItemsSource, Mode=OneWay}"
          Width="200" Height="30" />

Code behind:

public MainPage()
{
    this.InitializeComponent();

    DataContext = new TestViewModel();
}

And a very simple definition of the TestViewModel, using strings:

public class TestViewModel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    private IEnumerable<string> _myItemsSource = new List<string>
        {
            "Test Item 1",
            "Test Item 2",
            "Test Item 3"
        };
    public IEnumerable<string> MyItemsSource
    {
        get { return _myItemsSource; }
    }

    private string _mySelectedValue = "Test Item 2";
    public string MySelectedValue
    {
        get { return _mySelectedValue; }
        set
        {
            _mySelectedValue = value;
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs("MySelectedValue"));
            }
        }
    }
}

Now I thought this simple solution should just work... But when I start the app, the SelectedValue="Test Item 2" doesn't show up, the ComboBox is left empty. By setting breakpoints I noticed that the bound values MyItemsSource and MySelectedValue are corectly retrieved from the View Model when I set the DataContext of the view. After this action, the ComboBox.SelectedValue property is actually set to "Test Item 2", but it just doesn't show! Also I noticed that when I change the selected value in the ComboBox by user action on the UI, the changed value shows up in the ComboBox and the View Model property is updated accordingly. So everything seems to work fine except the initial visualization of the MySelectedValue View Model property. I'm becoming really desperate about that...

Now while this is the simplest example, in the origin I wanted to bind whole entities to ComboBox, setting DisplayMemberPath and SelectedValuePath. Unfortunately, the same problem occurs.