So what about Britain? Over here we have a far more tangled set of rules and regulations, thanks to the intervention of the European Union. The relevant EU directive essentially says that breaking "technical protection" (i.e. DRM) on software is fine, as long as you're not doing it for the purposes of infringing copyright (i.e. playing pirated games).
But that's Europe. In the UK, we have to ratify all of these EU directives before they become law, so the British lawmakers transposed the EU directive into British law, in the "Copyright and Related Rights Regulations 2003". That says essentially the same thing -- that it's okay to break these "technical measures" for interoperability reasons, but not for the purposes of infringing copyright.
The difficult bit comes when you try and interpret "interoperability". I spoke to Andres Guadamuz, a lecturer in IT law at the University of Edinburgh, who pointed out that there haven't been any test cases to set any legal precedent in the UK.
While there have been two cases of a ruling against people selling the tools to hack videogame consoles, these were predicated on the basis that the primary motivation to do so is to play pirated games. A similar, more recent, ruling outlawed so-called "R4 cards" in Britain, which are frequently used to play pirated Nintendo DS games.
But while piracy can be accomplished by jailbreaking an iPhone, the majority of people don't jailbreak for that reason. They jailbreak so they can get access to software created by third parties that Apple hasn't, or won't, put in its own app store. They jailbreak so they get more access to the device's hardware, and use it in different ways. They jailbreak because they don't want Apple's controlling hands on everything that they do on their device.
That's where the interoperability aspect of the law comes in. "The interoperation is between (third party) software and the iPhone's hardware,"