Several readers with access to the developer's release of iOS 4 have written to say that they can download iBooks for iPhone. In fact, most users report that they're being prompted to download the app. Just like the iPad version, it includes a free copy of Winnie The Pooh (to the delight of developers' children no doubt).
It seems that the app is restricted to 3GS iPhones, confirming what Engadget reported earlier. It's also notable that most report the app can only be downloaded via iTunes for the desktop and not over the air.
Enjoy reading about Pooh Bear, and imagine how beautiful the Hundred Acre Wood will be on that Retina Display.