I have read and put into practice everything here, an Apple doc on launch images.
My app supports auto-rotation and according to the document above, if you specify a Landscape launch image for iPad, it will be used when the app launches in landscape mode.
But I have never seen this actually happen in practice. The iPad can be oriented landscape, but the Portrait launch image always shows. This has been my experience on iOS 4 and 5 and on several different apps.
What is necessary to get a Landscape launch image to actually appear?
On your target settings, look at the iPad deployment info. Ensure that you have selected the supported device orientations that include landscapes. Then under the launch images, have you dragged your images into that field? If not, do that. I’ve never had a problem.