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Apple has filed a patent application on a technology for tethering rechargeable devices (like iPods) to particular chargers. The idea is that the device will only allow its batteries to be recharged if it is connected to an authorized charger.

Whether this is good for consumers depends on how a device comes to be authorized. If “authorized” just means “sold or licensed by Apple” then consumers won’t benefit – the only effect will be to give Apple control of the aftermarket for replacement chargers.

But if the iPod’s owner decides which chargers are authorized, then this might be a useful anti-theft measure – there’s little point in stealing an iPod if you won’t be able to recharge it.

How might this work? One possibility is that when the device is plugged in to a charger it hasn’t seen before, it makes a noise and prompts the user to enter a password on the iPod’s screen. If the correct password is entered, the device will allow itself to be recharged by that charger in the future. The device will become associated with a group of chargers over time.

Another possibility, mentioned in the patent, is that there could be a central registry of stolen iPods. When you synched your iPod with your computer, the computer would get a digitally signed statement from the registry, saying that your iPod was not listed as stolen. The computer would pass that signed statement on to the iPod. If the iPod went too long without seeing such a statement, it would demand that the user do a synch, or enter a password, before it would allow itself to be recharged.

How can we tell whether a DRM scheme like this is good for users. One sure-fire test is whether the user has the option of turning the scheme off. You don’t want a thief to be able to disable the scheme on a stolen iPod, but it’s safe to let the user disable the anti-theft feature the first time she syncs her new iPod, or later by entering a password.

We don’t know yet whether Apple will do this. But reading the patent, it looks to me like Apple has thought carefully about the legitimate anti-theft uses of this technology. That’s a good sign.