I've been looking around for an actual image of the iPad desktop (or the home page) in landscape mode. I couldn't find one which makes me wonder if the wallpaper will rotate at all?
If you look at the portrait image of the ipad desktop:
... like iphone, the desktop will be full with icons. It would be weird for those icons to move around.
I think, just like iPhone, the wallpaper won't rotate to keep perspective align with the icons. Others like apps, images, etc will rotate.