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APAC = Asia/Pacific, MEA = Middle East/Africa

FORTUNE — The next time someone tells you that the market for Apple’s (AAPL) iPhone is saturated, think of the chart at right posted Wednesday by Asymco‘s Horace Dediu.

At last count, he writes, the iPhone was being sold by only 30% of the world’s 816 mobile phone operators. Some of those carriers are bigger than others, of course. But all told he estimates that the iPhone is available for direct sale to only about 50% of the world’s 6 billion mobile subscribers.

Google’s (GOOG) Android, by contrast, is available to nearly 100% of those subscribers.

Where does the iPhone have the most room to grow? According to Dediu, the top 5 countries with unaddressed subscribers are

  1. India: 658 million
  2. China: 622 million
  3. Pakistan: 116 million
  4. Russia: 102 million
  5. Nigeria: 89 million

But the iPhone also has room to grow in the developed world. Dediu estimates that Japan is a 69-million-subscriber opportunity. Even the U.S. has 80 million subscribers without direct (i.e. subsidized) access to the iPhone.

The reasons Apple has half the addressable market of Android — or, for that matter, Research in Motion (RIMM) — are complex. They revolve, Dediu explains, around the question at the heart of Apple’s iPhone business model:

Can enough units be shipped in a sufficiently short time frame to allow a limited (but large) subset of operators to create competitive advantage which sustains a generous up-front subsidy.

The issues Dediu raises in The iPhone Addressable Market are among of the topics to be explored at his first Asymco conference in the U.S. It’s scheduled for Jan. 30 in San Jose, Calif. See Asymconf California for more details.