California legislators have killed a proposed "kill-switch" law, which would have required cellphone vendors to install software that "would render a phone inoperable if stolen." There are already a number of free apps that will do the trick---I use one of the oldest, Find My iPhone, that allows the owner to lock and/or erase his stolen iPhone, iPad, and Mac.
Frankly, I'm opposed to another law that presumes that government knows what's best for consumers. Let selection of such an app be voluntary, not mandatory; there's always the risk that the kill-switch could be triggered accidentally by the user or her kid, for instance. In an industry notable for rapid change, it would be foolish to pass a law that would likely stifle innovation. [Digression: how does this stifling happen? Often by prescribing a method to solve a problem---for example, everyone is required to wear a seat belt while riding or driving. The real objective is passenger safety, and out-of-the-box inventors are constrained by the seatbelt requirement.]
Besides, lockdown software solutions protect against the misuse of a phone after it's been stolen, not against the loss of the hardware itself.
American ingenuity has come up with a way to safeguard not only the phone but the person of the owner. Spraytect produces a pepper-spray system ($39.99) for the iPhone and Yellow Jacket makes a stun gun ($149.99) case. A kill switch won't be needed if you have either one of those.
[Note: illustrative flash video below won't play on iPhones or iPads]: