Jailbroken iPhones/iPads Will Lock Down The iBook Store
It seems that before opening a DRMed book, iBooks drops an improperly signed binary, tries to execute it, and if it works concludes that the device is jailbroken and refuses to open the book.
This apparently has been happening for a while now but only for certain apps and the protection is easily thwarted by a newer jailbreak that replaces the current Greenpois0n hack.