Apple could now be rethinking the benefits of  its exclusive deal with AT&T.

The iPhone dropped to third, behind RIM's Blackberry and Google's Android operating systems, in the race to become the U.S. smart-phone king, a Long Island research firm reported Monday.

The NPD report says that BlackBerry's operating system has 36 percent of the mobile market, compared to 28 percent for Android devices and 21percent for the iPhone.

But the iPhone's "decline" in overall numbers is due to the fact that its service is only with  AT&T.

The Android and BlackBerry phones have different manufacturers that produce dozens of different models available across many networks. Motorola, HTC and Samsung are just some of companies producing phones for Verizon, Sprint, T-Mobile and AT&T.

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Verizon, the largest U.S. wireless operator, has helped fuel success for the Android. It has launched a blitz campaign of its Motorola Droid and HTC Incredible. It has also been including Android devices in its buy-one-get-one free smart-phone promotion.

Despite the onslaught in operating system competition, Apple's iPhone is still very popular. In 2010 alone, Apple reported that it sold 8.75 million units, more than any other individual model using BlackBerry or Android operating systems.

Due to its popularity and impact on American pop culture, the next generation iPhone, rumored to be due out sometime this fall for the Verizon network, could easily overtake Android and possibly BlackBerry in the last quarter of 2010.

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