What does it mean to unlock a mobile device?

There currently are two technologies used by cell phones in the U.S.

In a locked phone that uses GSM, a software program will allow the phone to work only with the original SIM card. Unlocking the phone - which can be as simple as entering a code on the keypad - removes this restriction.

A CDMA phone cannot be unlocked to be used with another carrier. However, recent phones such as Apple's Iphone 5 contain both technologies. Therefore, an Iphone5 using Sprint's CDMA technology can be unlocked, allowing it to access GSM carriers.

Technology vs. Corporate Policy

CTIA - The Wireless Association has listed a set of guidelines for wireless companies to use in deciding whether to approve your request to unlock your phone. The guidelines, however, leave much much of the discretion and control of the process to the carriers, rather than the owner of the phone.

One example of how this can work against your interest comes from Sprint. The carrier has recently argued that its Iphone 5 can be unlocked for international GSM use only, while Apple claims the phones can be unlocked for domestic use also.

To read more about the CTIA guidelines and how wireless carriers hope to implement unlocking, see this Washington Post story.