If you buy an iPhone 4 or 3GS from Apple in the US or another country where the iPhone is only available on a single mobile network, that iPhone will only work with the carrier in question. But if you buy one directly from Apple in France, Canada, or the UK (at unsubsidized prices), it will work with any compatible network worldwide.

Unlike in the US, where "iPhone requires a two-year AT&T wireless service contract," iPhone 3GS and iPhone 4 buyers in the UK and France can get an unlocked iPhone directly from Apple itself. They can then sign up with any carrier that offers a compatible contract. But that's not all. The UK Apple Store page reads as such:

It works internationally.

Because this iPhone is SIM-free, you can purchase a micro-SIM or SIM card and service from a local carrier at your destination. Or check with your home carrier regarding international roaming charges.

The French and Canadian stores have text to the same effect; if you buy an iPhone from Apple in Britain, France, or Canada, you can use it with compatible mobile networks anywhere in the world. This makes it possible to change mobile operators within the country or use a local one while traveling, avoiding hefty international roaming charges.

This freedom does come with a price, though: in the UK, the flagship 32GB iPhone 4 costs no less than £599, the 16GB version £499, and the 8GB iPhone 3GS £419. In France, the prices are €739, €629, and €519, respectively. This is roughly $885, $737, and $619 for the UK iPhones and $907, $772, and $637 for the French ones—including VAT/sales tax. In Canada, there are no prices listed yet, but: "iPhone may also be available at a reduced initial price with a contract directly from your wireless carrier."

Buying an unlocked iPhone directly from Apple may or may not make sense: the phone is much more expensive, but it may be possible to get a cheaper plan. In the past, of course, mobile operators have been very successful at selling "cheap" subsidized phones with expensive plans. Orange in the UK, for instance, offers a range including one for £35 per month, which reduces the price of the 32GB iPhone 4 to £219 with a two-year contract. This plan comes with 600 minutes and "unlimited" mobile browsing—limited to 750MB per month.

It's also possible to get a SIM-only plan for £20 with 500 minutes and 500MB. With the two-year contract, the total costs add up to £1,059, while two years worth of the SIM-only plan and the unsubsidized 32GB iPhone 4 add up to £1,079. But with the SIM-only plan you get to change plans when cheaper ones become available—even from another mobile operator.

For those of us who reside in countries where Apple still has an exclusive deal with a single mobile operator, ordering from Apple in Canada, France, or Britain isn't going to work: "Orders placed through this Apple Online Store can be shipped only within the country of purchase." Then again, that's why we have eBay.