I don't have to give you a reference for this well-known story: the excellent woman in charge of making governmental exceptions to the draconian DMCA (Digital Millenium Copyright Act) has ruled that it's permissible for iPhone users to "jailbreak" their phone: that is, to install the OS of their choice if that enables them to run the programs they want. This remarkable decision makes sense if you focus on what the DMCA intends to protect. The DMCA appears to make it illegal to reverse engineer any software in order to break or hack said software. But the goal of the DMCA is to prevent such analysis in order to prevent the copying of copywritten material. Apple is taking advantage of the DMCA in order to patrol the apps that can appear on iPhones.
Perhaps what Apple is doing is similar to what HP did when the DMCA became law: HP put a computer processer in their ink cartridges, and when their competitors reverse-engineered how the printers talk to the cartridges, HP sued that they were breaking the DMCA. HP lost this case, because (as I understand it) there was no copyright issue involved; HP had just contrived a way to design hardware to try to take advantage of the DMCA rules. Now if an iPhone user avoids using almost all of the Apple OS in order to jailbreak the phone, where is the copyright violation?
But this DMCA "exception" does not promise to make life easy for the jailbreakers. They are permitted to modify the iPhoine software, but I think Apple is still permitted to change it back at will, in order to put the jailbreakers at a disadvantage. Apple just has to have a reason for forcibly downloading new versions of their iPhone software, other than just punishing the jailbreakers.
Expect frequent OS upgrades from Apple.