In a rivalry that should only grow more heated in the months to come, Research in Motion’s BlackBerry Curve moved past Apple’s iPhone in the first quarter to become the best-selling consumer smartphone in the U.S., research group NPD said on Monday.

RIM had three of the top five best-selling consumer smartphones in the period, with the Storm at No. 3 and the Pearl at No. 4, NPD said. T-Mobile’s G1 ranked No. 5.

NPD credited a “buy-one-get-one” promotion by Verizon Wireless for the Curve’s push past the iPhone.

“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,” NPD’s Ross Rubin said in the release.

RIM’s consumer smartphone market share climbed 15 percent from the previous period to nearly 50 percent in the first quarter, as Apple’s and Palm’s share both fell 10 percent. But don’t be surprised if those numbers change soon. Apple is widely expected to unveil a new iPhone in the coming months, while Palm’s highly-anticipated Pre smartphone is set to launch some time in the second quarter.

The smartphone market as a whole continues to grow, even as the larger handset market stagnates. The devices made up 23 percent of U.S. handset sales in the first quarter, up from 17 percent in the year-ago quarter.