I had to dig a bit to find this information when adding iPhone 5 support to my app. It’s pretty easy: you need to supply a 640px by 1136px launch image. That’s it. Of course, there may be layout issues that need tending to afterwards, but that’s a different story.
I suspect that Apple forced the inclusion of the launch image to make all devs use them. I know quite a few apps that don’t have launch images, even though Apple’s HIG says they’re required.