No. 1: Verizon iPhone

Android's greatest threat is that the iPhone moves to Verizon ( VZ).

AT&T and Apple had access to a large swath of the market, but the biggest portion, some 92 million people, are at Verizon. Apple needs that new market to boost sales.

Verizon has forged a friendly partnership with Google to push the Droid campaign. HTC and Motorola ( MOT) have supplied five Android-powered phones to Verizon and nearly all of them have been sellout successes.

But Android has been seen as the iPhone alternative. If Verizon has the iPhone, it won't need as strong an alternative.

--Written by Scott Moritz in New York.

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