As part of the 2009 DMCA rulemaking process, the Electronic Frontier Foundation filed an exemption for iPhone jailbreaking, something which hundreds of thousands of iPhone owners have already done. There are several applications that require jailbreaking such as turn-by-turn directions, using the iPhone camera for video, laptop tethering, and so on.
However, jailbreaking requires modified versions of Apple's bootloader and operating system software, which indeed is a breach of copyright once these modifications are distributed to third parties. As such, Apple has filed comments to the EFF exemption request, asking the Copyright Office to disregard it. However, as the EFF notes: "The courts have long recognized that copying software while reverse engineering is a fair use when done for purposes of fostering interoperability with independently created software, a body of law that Apple conveniently fails to mention."
So, for the first time, Apple has taken an official stance against jailbreaking your iPhone. It will be very interesting to see what the Copyright Office will decide.