Apple has warned that "jailbreaking" (i.e. unlocking) iPhones is not only a federal offense under US digital copyright law, it's also a grave threat to cellular networks.
The claim originates from a case filed by the Electronic Frontier Foundation to the US Copyright Office to legalize jailbreaking, arguing that users have the right to hack their own devices to run apps not authorized by Apple.
Apple filed a response claiming that jailbreaking an iPhone makes its baseband processor vulnerable to hackers, who could alter the code to initiate, say, a denial-of-service attack on a base station, thus crashing the BTS software and shutting down the tower.
EFF attorney Fred von Lohmann isn't having any of it, according to Wired's Threat Level blog, calling it a "theoretical threat" that has never actually happened despite over a million jailbroken iPhones at large in the US.
He also added that if forcing open a closed system posed such a danger, then open-source phones like the Android G1 are already a major threat.