From the iPad features on Apple’s web site:
The iBooks app is a great new way to read and buy books. Download the free app from the App Store and buy everything from classics to best sellers from the built-in iBookstore.
A lot of people (including myself) missed this from the Apple event. This is an interesting development as it means that iBooks will be on equal ground with other e-book readers in the App Store.
Not only does this mean that Apple will be able to update iBooks much more frequently than if it were tied to OS updates, it also means that Apple will have a fantastic example of in-app purchases to show off to developers.
It also leaves the door open for iPhone and iPod touch support in the future (if not at launch).