As it appears portrait upside down screen orientation doesn't work in Xamarin.Forms, and this is a show stopper for my project!
Here is the video that demonstrates this: http://www.screencast.com/t/yrqcXtVd3
(under simulator, but this also happens on the actual iPhone 6)
If anyone from Xamarin reading this please pay attention to this issue. I posted bug report: https://bugzilla.xamarin.com/show_bug.cgi?id=28786