Now, I understand why the new iPad can last the same hours as iPad 2 or the original iPad. It has a super large battery! In fact, based on the information, the new iPad has a built-in lithium-polymer battery of 42.5 Wh while the iPad 2 has just 25 Wh.
Here is a picture from iFixit on the battery.
I can imagine how light and thin the iPad will be without these batteries.