iPhone-mania swept through the United States last summer, as Americans camped out in front of Apple (AAPL) stores and shelled out hundreds of dollars to get dibs on the just-launched device. A year later, over 5.4 million iPhones have been sold and a 3G version of the device is expected to be unveiled within days. What’s more, the sleek touchscreen will soon test the waters overseas — at least 70 countries will start selling the iPhone later this year.

Apple hopes that international reach will help it reach its goal of selling 10 million iPhones by the end of 2008. But it could soon find out that getting the rest of the world to go gaga for its cell phone — however cool and flashy it is — will prove much more difficult than here at home. Here’s why:

– There is only one iPhone. Different wireless markets have different tastes and needs. That’s exactly why companies like Nokia (NOK), Motorola (MOT) and Samsung pour money and manpower into researching and developing a variety of phones with features, colors, form factors and price points that cater to each region and demographic. But there is only one iPhone — one iconic design, one interface and, at least as of now, a very limited price range. That could make marketing and selling the phone in countries like Japan, Finland, Mexico and India (all very different markets that are expected to begin selling the iPhone later this year) challenging, to say the least.

– The price tag. The iPhone currently sells for $400 to $500 in the United States. But carriers in the United Kingdom and Germany (two out of three other countries that are already carrying the device) have already had to lower the iPhone’s price. In developing markets, the high price tag might be even more of a barrier for consumers. Lucky for Apple, there is a growing middle class in many of these countries.

– iPhone clones are everywhere. Last year, when the iPhone first entered the U.S. market, it was the only phone of its kind. These days though, the revolutionary devices is no longer, well, revolutionary. Taking cue from the Cupertino, Calif.-based company, other phonemakers like Samsung, Sony Ericsson and LG are flooding the market with their own versions of all-touchscreen devices. That means that consumers abroad can opt for an iPhone lookalike — some of which are cheaper and have already been around for months — instead of the real thing.

– Other countries are more advanced. When it comes to wireless technology, the U.S. is way behind. Even if Apple makes a 3G iPhone — one that would work on faster networks — available overseas, it’s not clear just how many advanced features new versions of the device will include. In countries like Japan and South Korea, consumers are used to watching live television and paying for subway passes with their cell phones. It’s unlikely the iPhone will be able to do all that anytime soon.