What you and the other poster, yode, are arguing is the common wisdom about this sort of thing. You, however, are not arguing what is the law. That is why I included the harvard link to the actual law so you can read it and understand that DMCA in fact DOES make it illegal to do things with "your own" hardware/sfotware. Reverse engineering is illegal. That includes reverse engineering software that you legitimately purchased, even if you do not disseminate the modifications. So, software, modified for your own purpose and using your own hardware is ILLEGAL. Only very particular circumstances allow for lawful reverse-engineering, such as if you have received written consent from the copyright holder and you needed to reverse engineer the software in order to get your software to work with theirs, NOT to cause their software to behave in an unintended way or to break any kind of protections.
Don't let the absence of a high profile conviction fool you, computer crime is being taken very seriously. Apple has the legal right to control what is on their iPhone product, weather you like that idea or not. This allows for very high stakes exclusive contracts, like with AT&T, that would not be possible if jailberaking/unlocking your iPhone was legal, meaning other carriers (T-mobile for instance) could legally jailbreak, unlock and SELL iPhones in their stores. Companies look to capitalize in whatever fashion they can. If companies aren't unlocking iPhones to make more money that's a good indication that it's ILLEGAL.
Sorry to pop your naive legal cherry on this topic but the internet and your computer is not the legal free for all you want it to be.