Unlocking an iPhone used to be a major sin against Apple and AT&T, at least in the U.S. But, late last week, AT&T announced that it would now help owners of older, out-of-contract iPhones to unlock them, starting on Easter Sunday.

AT&T said in a statement that the only requirements are that the user's account must be in good standing, the device cannot be associated with an active AT&T account, and the customer Relevant Products/Services "needs to have fulfilled their contract term, upgraded under one of our upgrade policies or paid an early termination fee."

IMEI Number

Given those conditions, AT&T's offer applies to the earlier iPhone models -- the original one, the 3G, 3GS, and 4.

To unlock such a model, the user provides the IMEI, or International Mobile Equipment Identity, number at an AT&T store or over the phone, receives an e-mail from Apple with the instructions and reference codes, and follows the steps. The IMEI is found in Settings>General>About.

A specific account can only receive up to five unlock codes in a year. To unlock a phone currently on an iPhone contract, an early termination fee is required.

Unlocked iPhones will work on T-Mobile's network, but at slower-than-3G data Relevant Products/Services speeds. When traveling abroad, users can get onto a local GSM wireless carrier via a new SIM card. Older, unlocked iPhones will not work on CDMA-based Sprint or Verizon networks.

Verizon and Sprint both allow their iPhones, even under contract, to be unlocked for use when traveling in other countries, which could be one reason why AT&T has given in. Apple sells unlocked iPhones, which it has done for some time in Europe and other markets, and has been doing in the U.S. since last summer.

A Pricey iPod Touch

Locked phones are bound to a given carrier's network, while unlocked phones can be adapted to work on another network with a new SIM card. Apple and AT&T have, in the past, been particularly hard-nosed about not supporting unlocking of the iPhone, although users have jailbroken the devices on their own. However, AT&T has been less reluctant to unlock other brands that it carries.

Unless an iPhone can be unlocked, a customer who has upgraded to a newer phone is left with a locked iPhone that has become, essentially, a pricey iPod Touch. Some observers are noting that this new support for unlocking older devices may be appealing in particular to businesses, which might have multiple, older models sitting around.

There could be a concern, of course, that might particularly affect business Relevant Products/Services users of unlocked phones: security Relevant Products/Services. Jailbroken phones have generally been considered full of security issues, but IT departments may consider officially unlocked phones less suspect. Or maybe not.

Another advantage for both consumer and business owners of older iPhones is that their possessions, which now have legitimate second lives, will likely become more valuable on the open market.