It appears that last week’s update to iBooks includes some form of jailbreak detection. According to comex, the new iBooks attempts to run improperly signed code, and if it succeeds, the app refuses to open DRM’d books purchased through the iBookstore. The basis of jailbreaking is to allow devices to run unsigned code, so Apple’s approach is very effective.
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.
In order to go back to reading, users will have to restore their device, and not jailbreak. It’s unknown if any developer is working to fix this issue, but for the time being, don’t update iBooks to the latest version (if you haven’t already). It’s also possible that Apple plans to sneak this code into other 1st party apps.
If you are currently jailbroken and have purchased a book from the iBookstore (and are up-to-date), let us know in the comments if you are able to open the book, and what jailbreak/firmware you are running. If iBooks detects a jailbreak, the following error message will appear: “There is a problem with the configuration of your iPhone. Please restore with iTunes and reinstall iBooks.”
Update: The nasty code has apparently been lying dormant for a while now, but the latest greenpois0n cracks open a wide enough hole to trigger the jailbreak detection. Comex has developed a fix for the issue, and it should make its way to the public shortly.
Update 2: The fix has been implemented in PwnageTool 4.2.1, and should be integrated in the next version of greenpois0n.