It's been about five months since Apple's iPhone debuted on Verizon's network and already it's the carrier's top-selling handset. This according to BTIG Research (registration required), which spent the past three weeks asking 250 Verizon and AT&T stores across the United States to name their No. 1 smartphone.

Of the Verizon stores surveyed, 51 percent said the iPhone was their top seller, and an additional 38 percent said it was tied for first with a rival Android device, typically the Samsung Droid Charge or the HTC Thunderbolt.

At AT&T stores, the iPhone was even more dominant--65 percent of them named it as their top-selling phone. Another 31 percent said it was an Android device.

Keep in mind the iPhone 4 is very nearly a year old.

So all told, 58 percent of the stores BTIG called named the iPhone as their strongest-selling phone, and an additional 20 percent said it shared that honor with a competing device.

That is astonishing, really. The iPhone debuted in 2007, and the first Android phone shipped in 2008. Five years later the two are slaughtering incumbents like RIM, whose BlackBerry was rarely mentioned by respondents. Said BTIG analyst Walter Piecyk, "At AT&T, there were actually a handful of stores that mentioned a BlackBerry and a Windows Phone as the top seller in their store. We never got that response from any of the Verizon stores, which have dated versions of the BlackBerry."

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(Credit: BTIG Research)