I'm a lawyer, but it would be unethical for me to render a legal opinion on a forum like this. Having said that, analzye this yourself: Under what basis could Apple or ATT claim that by purchasing the iPhone you are obligated to use ATT's service?
1. Contract law. I have seen nothing in any of the terms of service that require me to use ATT. I certainly did not sign anything saying I would agree to use ATT when I bought my iPhone. The fact that I KNOW that Apple expects me to use ATT when I activate my iPhone does not legally obligate me to use my iPhone with ATT.
2. Codes. There is no statute or code that requires you to use your cell phone with a particular carrier. There ARE statutes that provide protection against copyright infringement or trade secrets theft. The DMCA was passed in the US to try and prevent people from modifying certain digital items that might be covered by a copyright. The Library of Congress has issued an exemption to the DMCA that allows cell phone owners protection if they modify their cell phone to unlock it. While that exemption provides some protection to cell phone owners, it might not provide protection to someone selling or offering a service to unlock other people's phones. But since the exemption was directed towards the end-user, it would be hard to see Apple or ATT bringing a legal action against end-users (i.e., iPhone owners) to prevent them from unlocking their iPhone. Of course, the DMCA is a US law. There are a whole bunch of other countries out there, and in some of them, people have the right to have their cell phone unlocked under certain circumstances. Since you could always then buy such an unlocked iPhone and take it with you wherever you wanted (e.g., the US), owning or using an unlocked iPhone can't be illegal per se, under any statute or code.
3. Legal decisions/caselaw. To my knowlege there haven't been any yet with respect to the iPhone. It's too soon, and it's hard to see when such a dispute would ripen to a full-blown legal decision. It is MUCH easier for Apple/ATT to try and close the loopholes that allow unlocking (they have the right to do this), and then entice unlocked users to re-lock their iPhones with stunning new features (that's just an exercise of free choice by the iPhone user). If Apple/ATT are going to spend good money on addressing this issue, they will probably spend it there. BUT, they will also take steps to protect what copyright rights they clearly do have. One person who is looking to hack the iPod Touch was planning on posting some of Apple's code on a secure portion of his website to allow select others to access it. Apparently Apple hit the ISP with a cease and desist request, and the code was taken down, since it may not be legal under the DMCA to distribute Apple's proprietary code like this.
Possible conclusions: Is unlocking the iPhone a slightly grey area? Yes. Not because the end user has signed away or waived his/her right to use their iPhone on another network, but because the process of discovering an unlock might run afoul of the DMCA, and because the practical life of an unlock solution might not be satisfactory to many people. Still, it would be hard to see end users being prosecuted or legally challenged for using hacked or unlocked iPhones. The DMCA exemption is there and it was intended to protect them. Prosecuting the hackers themselves is difficult, because if they're hacking to unlock their own iPhone, they are probably protected by the DMCA exemption, too. If the hackers share and exchange data that is protected by the DMCA they might run into trouble, but finding and prosecuting that, in my humble opinion, would be difficult if not impossible (going after someone hacking in the US is one thing, doing so for someone outside the US would be practically impossible in many situations).
This is why I think you see Apple taking the exact route they appear to be doing -- trying to close up known loopholes and trying to entice unlockers to re-lock.
