Of course, having your iBooks library on all of those different devices might make life confusing, but according to The Jobs, there will be wireless syncing of your books between platforms. Let's say you're reading a book on your iPad, and you're going to a restaurant so you decide to just pick up your iPhone and head out. When you get to the restaurant, you fire up the iPhone and bring up iBooks, and it's on the same page that you were reading on the iPad.
This is currently the way that Amazon's Kindle platform works, syncing books between a number of devices. I currently have Kindle Reader on my iPhone 3GS, iPad, and two Macs, and it is refreshing to be able to switch between devices and always have the current book and page available.
As with iBooks for iPad, a free illustrated copy of Winnie the Pooh will be provided with each copy of iBooks on the iPhone.