If you have noticed the PhoneGap Windows Phone Project , you will find a MainPage.XAML page where the PhoneGap runs .
By Default , the Windows Phone Silverlight Application runs in the Portrait mode . To support the Windows Phone PhoneGap application to support both the orientation ( Portrait and Lanscape ) , follow the below steps
Read more about Windows Phone and PhoneGap – Article #9 – Handling Orientation – Part 1 @WindowsPhonerocks.com