It's the screen brightness that is causing the battery to get hot. Turn the brightness down and you'll find the back to return to room temperature, or at least slightly warm. I'm not real happy about this as I like my brightness turned up all of the way.
I saw a Genius today who tried to take temperature readings, but said that there wasn't enough data yet for the iPad to report anything. He basically told me to wait it out a couple of weeks while they get new shipments in and return. He also said that I was the first person to report this in that store, as far as he knows. Being just the second day since its released I'm not surprised. Lastly, he said the next step was to restore the OS with iTunes - although I wouldn't hold out any hope of that making a difference.
Long story short - turn down the brightness while we wait. I'm hoping it's only affecting some iPads, and not all.