This is also an issue for one of my apps, which can launch in a horizontal landscape, there's no way to create two Default.png files, once for each orientation (correct me if I'm wrong, would love to optimise for it).
You can (and would) on iPad, because apps can launch in any orientation. On the iPhone, apps always launch in portrait orientation (unless it's a landscape-only app) so your iPhone launch image only needs to support a single orientation.
