She told us that an author creating an app for the iPad, especially a visually oriented one, will usually design it for one orientation only. This is because double the imagery's needed for dual orientation support and that creates an overhead in the app's size.
Adobe
One major advantage to portrait-only or landscape-only folios is that you have to create only one design. If you have a printed magazine, converting the layout to a 768×1024 page size isn’t nearly as difficult as converting it to both a 768×1024 and 1024×768 page size.Source: http://blogs.adobe.com/indesigndocs/2011/02/design-decisions-for-digital-publishing-apps.html
More will be covered on iPad screen orientation in a following post.