Apple has finally started selling unlocked iPhone 4s in its US stores, which means that the popular smartphone is now sold micro-SIM-free and can be used in Australia and abroad.

It's a great deal for Australians travelling through the USA with the strong Australian dollar.

Although iPhones sold in Australian Apple Stores are unlocked they cost up to A$300 more than their US counterparts. But previously, US iPhones were locked to the AT&T network, which prevented them from being used in Australia or indeed any other country.

Now you can pick up an unlocked iPhone 4 on your travels, pop a micro-SIM card into it, and use it in Australia and around the world.

And unlike many smartphone brands, even though you bought the phone in the US you're still covered by Apple's international warranty.

The price: hundreds of dollars cheaper

At US Apple Stores, you'll find the 16GB iPhone for US$649 and the 32GB version for $749. That's A$606 and A$700.

In comparison, the same phones are A$859 and A$999 in the Australian Apple Store.

Of course, you'll have to pay the local sales tax (like a state-by-state and county-by-county version of GST). And unfortunately, there's no sales tax refund system for visitors like there is in Australia, Europe and elsewhere.

Five states -- Alaska, Delaware, Montana, New Hampshire and Oregon -- don't have a sales tax. So if you're passing through them, pick your iPhone (and other large purchases) up there.

California -- where most Australians connect to return home -- has the highest sales taxes in the country, with an 8.25 percent state tax and up to 10.75 percent additional local tax depending on city and county.

Texas, home to Qantas' new Dallas/Fort Worth flights, is cheaper, with a 6.25 percent state tax and up to 2 percent local taxes. (There is a Tax-Free Shopping Weekend August 19-21 this year, though!).

New York City, another popular destination, has a 4 percent state and 8.875 percent city sales tax.

Washington DC has a 6% tax, while Virginia (where Dulles airport is located) is lower at 5%. If you're planning a trip to Delaware (a couple of hours' drive away), there's no sales tax there, which could make an evening or weekend trip economical if you're buying an iPhone and a laptop, say.

So you'll need to budget on an 8-20 percent tax on top of the US prices, but you'll still stand to save against buying your iPhone 4 in Australia, and if you want you can pick up a pre-paid US SIM and start using your iPhone right away.

Not travelling to the US?

If you're not headed stateside and want to import an unlocked iPhone 4 to Australia you can use PriceUSA. This Australian-based company has a purchasing agent in Oregon who buys items for you, sales-tax free, and then ships them on to Australia.

PriceUSA adds its own 5% fee on all sales and you also lose a bit in international shipping. But as you can see from the PriceUSA quotes below there's still a considerable saving over buying the phone in Australia -- you'll come out $153 ahead if you're buying a 16GB iPhone 4 (against the Aussie price of $859) and $188 on the 32GB iPhone 4 (which costs $999 here).

The "delivery protection" line item in the PriceUSA quote above is an optional thing -- it covers return postage to the USA if the wrong product is shipped by the vendor (which is unlikely in Apple's case).