Windows Phone back button bug?

Hi, in Windows Phone canceling the OnBackKeyPress event doesn't prevent backward navigation. I think this is either a bug, or a crappy feature:

Example (Windows Phone MainPage.xaml.cs):

protected override void OnBackKeyPress(CancelEventArgs e)
{
     e.Cancel = true;
}

Is there a workaround or preferred method for canceling NavigationPage.Popped in response to a back button press?

Thanks,
Gabe