Apple currently holds the top spot among smartphones available in the U.S. with last year's iPhone 4.

The company, according to a research, is also occupying the second most popular slot with its budget priced iPhone 3GS, which is currently available in the country on one carrier.

According to The NPD Group, a leading market research company, Google's Android operating system has continued to dominate U.S. smartphone market share, accounting for 52 percent of units sold in the second quarter of 2011.

Like Android, Apple's iOS has experienced slight quarterly gain rising to 29 percent in Q2; however, BlackBerry OS share has fallen to 11 percent, as Windows Phone 7, Windows Mobile, and webOS has held on steady at less than five percent of the market each.

The research has revealed that the most popular US smartphone is Apple's comparatively expensive iPhone 4, which was released over a year ago, followed by the two-year-old iPhone 3GS in the second place owing to its low price.

The iPhone 3GS is currently available on AT&T compared to the iPhone 4 which is available on Verizon as well. The iPhone 3GS is currently available for $49 with intermittent promotions that has made the device free with a contract. Refurbs of that model can go for as low as $9. Best Buy offered the phone for free with contract Monday.

A brand new 16GB iPhone currently costs $200 with a two-year contract and the 32 GB version sells for $300. Verizon, which began selling the iPhone 4 in February, doesn’t offer the 3GS iPhone as an option.

Google's acquisition of Motorola shifts the balance of power in the handset-patent conflict between Google and its operating system competitors, said Ross Rubin, executive director of industry analysis for NPD. Android's momentum has made for a large pie that is attractive to Motorola's Android rivals, even if they must compete with their operating system developer.

Apple is expected to release a new iPhone 5 around the end of September or early October, and is anticipated to similarly offer a lower end version of iPhone 4 as its budget-priced model, AppleInsider reported.