Apple has filed a motion with the US Copyright office claiming that the very act of Jailbreaking an iPhone is a violation of copyright and seeking a ruling under the Digital Millennium Copyright Act.
I am not at all surprised that this is happening in the US as it is one of the few places that Apple still has a very strong single carrier hold on the iPhone.
However the Electronic Frontier Foundation has filed a counter motion asking that Jailbreaking be given an exclusion saying it represents fair-usage to owners of the iPhone.
Apple's counter (and a pretty strong one) is that they current methods for jailbreaking involve making changes to the copyrighted bootloader and OS and those changes constitute the violation.
Apple has no plans to go after individual users at this time but are experimenting with software that only works if the Bootloader and OS are intact.