It's all a moot argument anyway. US copyright policy specifically allows the modification of mobile phones to allow them to run legally acquired thirty-party software.
http://www.copyright.gov/1201/
Maybe Apple would win a lawsuit against you on pure copyright (violation of exclusive rights versus fair use, not violations of DMCA anti-circumvention provisions, which as pointed out previously are separate issues), and maybe they would not.
In making recommendations for the Librarian of Congress, the Register of Copyrights explained her conclusion that she thought the simple act of modifying a phone's OS to allow legally obtained 3rd party programs to run "fits comfortably within the four corners of fair use" as a defence against exclusive rights violations -- and used that position as a premise to reach her recommendation that the DMCA exemption was worth supporting. But it would be outside the scope of their authority as rulemakers on DMCA exemptions, for either the Register of Copyright or the Librarian of Congress to actually make the fair use defence into an official policy.
They have the authority to remove the DMCA roadblock, which they have done. They don't have the authority to officially enact the fair use defence -- it's up to the courts to test the existing laws to see if they agree with the Register of Copyright's conclusions, or lawmakers to create new laws which specifically address the issue. (The courts do give some weight to the opinions produced by the Register of Copyrights when it comes to interpreting law, so there is good reason to believe that they would be likely to agree that the fair use defence is applicable.)
Regardless of whether or not the things you do with an iPhone after you jailbreak it count as fair use or not, the fact of the matter is you've still breached a contract with Apple. And after you've breached that contract, Apple is no longer obliged to provide you with the service they'd promised to give you under that contract. That could easily include causing iBooks to cease to function.
(Remember, contracts can, and regularly do, involve people promising not to do things that would otherwise have been perfectly legal. You could sign an endorsement contract with Reebok promising never to wear Nike products, in exchange for a nice fat paycheque. If it weren't for that contract, it would have been perfectly legal for you to wear Nike products -- but as long as you want to keep on getting that paycheque, your contract prevents you from wearing Nike products.)
