Research In Motion's BlackBerry Curve outsold Apple's iPhone last quarter in the United States, according to data from the NPD Group.

According to the report, titled "Smartphone Market Update," RIM had a very successful quarter and had three of the top five best-selling smartphones for the first quarter. With the Curve at the top spot, the touch-screen BlackBerry Storm was the third most-popular smartphone, and the BlackBerry Pearl was in fourth place.

"Verizon Wireless' aggressive marketing of the BlackBerry Storm and its buy-one-get-one BlackBerry promotion to its large customer base contributed to RIM capturing three of the top five positions," said Ross Rubin, director of industry analysis at the NPD Group, in a statement. "The more familiar, and less expensive, Curve benefited from these giveaways and was able to leapfrog the iPhone, due to its broader availability on the four major U.S. national carriers."

While it may be difficult to replicate the buy-one-get-one free promotion in future quarters, the sales success shows that BlackBerrys are entering the mainstream. The company said more than half of its 25 million subscribers are now noncorporate users. RIM recently launched the App World to capitalize on this mainstream market.

Apple still had a strong quarter, as the iPhone 3G led to 1.6 million new subscribers for AT&T. Apple may get another sales boost this summer when it releases the iPhone 3.0 software, and many industry watchers expect the company to have a new smartphone as well.

The T-Mobile G1 was the fifth best-selling smartphone of the quarter according to NPD, and it has sold more than a million units since its release last October. The sales success is a good sign for the Android operating system, and it may spur other manufacturers to take a closer look at the Google-backed OS.

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