The nation's biggest carrier is acknowledging that, for the first time, it's selling an unlocked phone that can be used on a GSM network such as rival AT&T 's.

Not an Easy Fit

That would require some jury-rigging to make AT&T's SIM card fit in the Verizon iPhone's smaller card slot, however.

To do so, blogger Jeff Benjamin of iDownload cut his AT&T card, attached a piece of tape to it and jammed it in the iPhone with a paper clip -- something most users would be reluctant to do with their shiny new Apple toy, which begins at $199 for the 16-gigabyte version.

Once altered, the Verizon iPhone can access AT&T's voice network and its HSPA+ data network, but not its long-term evolution high-speed data network.

This means iPhone 5 users will be able to access GSM networks abroad with compatible SIM cards. But switching carriers won't be so easy when you factor in early termination fees from Verizon, which can be as high as $350 (depending on how far the contract has progressed at the time of cancellation.)

It could make it easier to switch to AT&T after contract, but judging from the gigantic lines at Friday's launch and previous launches, Apple fans tend to upgrade to the next model more than other device owners.

"The Verizon Wireless iPhone 5 is global GSM ready and unlocked but this freedom may come with caveats," said Neil Shah of Strategy Analytics. "Technically it should work on AT&T's network for GSM/HSPA+/LTE but I believe this could be relocked anytime, and using it for non-roaming purposes during the contract period might involve contractual penalties," he told us.

"Verizon has kept it unlocked to reduce the pain for its customers, thus boosting higher roaming revenues in the long run. However, in short-term, getting a nano SIM card won't be that easy for roaming purposes outside of U.S."

Verizon spokeswoman Brenda Raney told The Associated Press the company had no plan to lock the phones to keep them from being used on other carriers.

Locked Up

Switching other carriers' iPhones to Verizon won't be as simple because Sprint and AT&T lock their phones and will only unlock them for users who travel.

T-Mobile, the fourth-biggest wireless carrier by subscribers, is the only one that doesn't carry the iPhone, but it has been making a concerted effort to lure out-of-contract iPhone users to its network.

The iPhone 5 was launched last week and Apple says it sold 5 million of them in the first three days, surpassing the record of its last iPhone, but somewhat short of analysts' projections. Those figures, however, do not take into account online orders that have not yet shipped.

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