Amazon Kindle, meet your Voldemort. iPad has an ebook store and it's called "iBooks". Picture of the store interface after the jump. Publishers include Penguin, HarperCollins, Simon & Schuster, Macmillan, and Hachette—the big boys. [Apple iPad]
Doesn't look like books will be cheap, but there should be a healthy price war soon. The example book was $15.
On the plus side, Apple is using ePub, which means books from other stores could work.