Chalk it up to the fact that the Curve is available on the four biggest U.S. carriers, not to mention a two-for-one sale on Verizon Wireless. Also in the top five for the first quarter of 2009: The Storm and the T-Mobile G1.The NPD Group just released its U.S. smartphone sales numbers for Q1, and according to their numbers, RIM had BlackBerrys in three of the top five positions, and managed to topple the iPhone 3G from the top of the list.

Despite its early launch woes, the touchscreen BlackBerry Storm appears to be selling strongly, coming in a No. 3 on NPD's list behind the Curve and the iPhone 3G. 

After the Storm comes the ever-popular BlackBerry Pearlwhich, like the Curve, is available on all four major U.S. carriersfollowed by the Android-powered G1 on T-Mobile.

So, how did the Curve manage to overtake the iPhone 3G? Well, as the NPD group notes, availability on all the big carriers always helps (the iPhone is, of course, still only on AT&T), as well as its $99 (with a new two-year contract) price tag. Meanwhile, Verizon Wireless probably helped boost Curve sales with its "buy one, get one free" promotion, according to NPD.

And here's a potentially worrying trend for Apple: the NPD found that RIM's share of the smartphone market rose a solid 15 percent to a full 50 percent, while both Apple and Palm (which didn't land a handset in the top 5) both saw their shares fall 10 percent.

Of course, it could be that smartphone shoppers are holding off on both iPhones (given that a new model might be announced as early as next month) and Palm handsets (seeing as the red-hot Pre will likely go on sale in a matter of weeks).

Then again, never underestimate BlackBerry, which still has some of the best smartphones in the business and has price and availability on its side, whereas the cheapest, AT&T-only iPhone 3G remains at a somewhat lofty $199 (with contract).