Apple's iBooks have always been protected from running on unauthorised devices thanks to the company's FairPlay DRM. That is, until Requiem version 3.3 decided to throw a cow over iBook's walls.
Requiem is a popular app designed to remove the protections placed on music and movie content purchased from the iTunes store. However, the newest version of the program adds the ability to crack iBook's DRM as well. This allows the digital books to be converted into any eBook format and read on any applicable device, not just Apple's ePub platform. That means that if you decided, for some reason (you were struck by a cow or something, I don't know), that you wanted to ditch your iPad for HP's upcoming Win8 Tablet or a Kindle Fire, you'd be able to bring the books that your rightfully paid for along with you.
There is, however, a question of how long the crack will last before being resealed. FairPlay has long been under siege by Requiem and similar apps, so Apple routinely updates its DRM systems to counteract the programs' exploits. [Appadevice via Cult of Mac]
Disclaimer: Neither Allure Media, Gizmodo, nor the writer in any way condone cracking, hacking, exploiting, socially engineering, or otherwise bypassing Apple's FairPlay DRM system. Hence, I'm not including a link to the program -- you'll just have to spend the same three seconds I did finding it with a Google search.