A Copyright Office/Apple reality check
The exemption is welcome, but probably will make little practical difference to much of anyone. Although Apple strongly opposed the exemption, the company had never taken any action against jailbreakers, including the iPhone Dev-Team that has done most of the work. So the Copyright Office has said that Apple cannot do what it was not doing anyway.
More significantly, the exemption does not stop Apple (or other phone makers or carriers) from trying to protecting their products. Apple, for example, has forced the Dev-Team to write new code with each iOS release while some Motorola phones, including the Verizon Droid X, include an eFuse chip that can deactivate the phone if unauthorized system software is installed. These moves are unaffected.