Oops answered my own question - although perhaps it will help somebody else!
I just used this, and posted to my statusMessage div I'm using to debug stuff:
$(document).ready(function() {
$('#statusMessage').html(Windows.UI.ViewManagement.ApplicationView.value);
if (Windows.UI.ViewManagement.ApplicationView.value == Windows.UI.ViewManagement.ApplicationViewState.fullScreenPortrait)
{
$('#statusMessage').html('portrait!');
}
});