With as much growth and popularity that Apple's (Nasdaq: AAPL) iPhone has seen, would you believe that there's still incredible room for even more upside in the years to come?

As Apple's most important business segment and biggest cash cow -- 43.4% of revenue last year -- all eyes are on where iPhone fever will spread next. Recent analysis from Morgan Stanley analyst Katy Huberty shows there's still plenty of money on the table.

As the company has expanded its carrier partnerships beyond AT&T's (NYSE: T) initial exclusivity, it has exploded iPhone unit sales from 11.6 million in fiscal 2008 (the first full year of iPhone sales) to 72.2 million in fiscal 2011. Apple's carrier list now counts 230 carriers in 105 countries, including Sprint Nextel (NYSE: S) and Verizon (NYSE: VZ) here in the United States.

Asia is the next frontier for the iPhone.

China represents an enormous opportunity once China Mobile (NYSE: CHL) joins China Unicom (NYSE: CHU) as an official carrier. The iPhone 4S has yet to make an official debut, although it draws near, as it has just recently passed regulatory muster. That hasn't stopped ambitious resellers from sneaking the devices into the country early and selling them for upwards of $2,000.

Beyond just China, Huberty indicates that 78% of Asian carriers have yet to offer the iPhone, representing 67% of the geography's subscriber base. Those between ages 25 and 34 see the highest smartphone penetration, and Asia has 655 million people in that category, or 10% of the world's population.

On the flipside, she also notes that a large proportion, around 69%, of mobile subscribers in emerging markets tend to be prepaid. This proves to be a challenge, as the vast majority of iPhone buyers rely on carrier subsidies and service contracts to cover the majority of the iPhone's hefty retail price tag, which now starts at $649 for an entry-level model in the United States.

Huberty brings up the possibility of a lower-priced model to address the lower end of the spectrum, saying that a $300 phone would more than double demand.

The 2-year-old iPhone 3GS is now already practically free on contract, and it could easily be the lower-priced solution for emerging markets that Huberty is asking for. Penetration for 3G in those areas is incredibly low, and as Apple starts to tap into the vast Asian market, the picture of a new era of growth starts to become clear.

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