Research In Motion's BlackBerry Curve has moved past Apple's iPhone to become the top-selling consumer smart phone in the US in the first quarter of 2009, according to market researcher The NPD Group.

The iPhone 3G fell to second place, as Apple's consumer smart phone market share dropped 10 per cent. Two more BlackBerry phones, the Storm and Pearl, held the third and fourth spots, respectively. And the Android-based T-Mobile G1 made an impressive fifth-place showing. Palm, which is expected to roll out its ballyhooed Pre phone later this month, also suffered a 10-per cent drop in market share.

Things certainly look rosy for RIM and its popular BlackBerry series. But it's doubtful the Curve, including all the 83xx models, will continue to outsell the iPhone.

Here's Why

The BlackBerry's impressive Q1 sales surge got a huge boost from Verizon Wireless's consumer-friendly "buy-one-get-one" BlackBerry promotion, as well as its heavy marketing of the iPhone-like BlackBerry Storm, which is selling like hotcakes despite tepid and occasionally hostile reviews from the tech press.

Verizon certainly won't continue its costly two-for-one giveaway forever, which should result in a slide in Curve sales. As for the relatively new Storm, its popularity will ultimately be determined by consumer word of mouth. If everyday users agree with the pros that the Storm doesn't cut it, sales will fall.

But why did iPhone sales fall 10 per cent? Rumours of a new and improved iPhone have been circulating for months, and it's likely that potential buyers are sitting on the fence to see what Apple rolls out this summer. When the new iPhone launches and the Apple marketing machine kicks into high gear, iPhone sales will certainly pick up.

Another competitive advantage for the BlackBerry is its availability on all four US wireless carriers, according to Ross Rubin, director of industry analysis at The NPD Group. Of course, we all know the iPhone is currently limited to AT&T Wireless. But reports say Apple may be cooking up a deal to bring the iPhone to Verizon Wireless too. If that comes to pass, iPhone sales should get another big boost.

Finally, the Palm Pre will certainly steal sales from RIM and Apple. But I suspect its physical keyboard will appeal more to customers who were leaning toward the BlackBerry Curve. We'll soon know for sure.

Note: This blog first appeared on our sister site PCWorld.com.