The difference is that one is legal and one is not. Hacking the iPhone is consistent with your full use and enjoyment of the product, so long as you remain lawfully accessing the provider's network and so long as you do not infringe on anyone's property rights in doing so. You are not making illegal copies or violating an explicit agreement. Putting OS X on a PC is a clear violation of license terms to the software, just as putting the iPhone OS on a Nokia would be.
Matticus, in my interpretaton, you've got it exactly backwards.
People who "hack" their iPhone willy nilly with the open-ended intent of installing any arbitrary 3rd party application are in material breach of Apple's EULA. As such, Apple, if it so chose, could sue all such people with copyright infringement, and would have a fair chance of winning.
People whose sole intent in jailbreaking their iPhone was to install a piece of SIM-unlock software, are arguably NOT in violation of the EULA. It is the the language of the EULA itself, in light of the DMCA exemption, that leads me to that conclusion. I've explained my reasoning previously in other threads, so I really don't want to get into it again if you don't mind.
If those SIM-unlocked people have already activated a service agreement with AT&T, then they are still contractually required to pay the early termination fee or else they'll still be on the hook for paying the prescribed monthly service fees, regardless of whether they go on to use it or not. But that's an issue that's up to AT&T to pursue.
If those SIM-unlocked people never activated their phones on the AT&T network in the first place, then they have not entered into a contract with AT&T, so AT&T has no standing in the matter. Only Apple's software EULA is applicable, and again, I hold that agreement has *not* been violated.
Now let me clarify - I am not convinced that Apple has any obligation to provide warranty support for any bricked iPhones that have been hacked in any way at all. I just don't agree that people who have SIM-unlocked iPhones have necessarily done anything
illegal.