By Philip Elmer-DeWitt
September 11, 2012

FORTUNE — The NPD Group’s Stephen Baker chose Tuesday 9/11 — the day before the expected launch of Apple’s (AAPL) new iPhone — to pour cold water on what he calls the “growing expectation that this release will achieve the equivalent blowout sales success that the iPhone 4s enjoyed.”

Equivalent? As we reported Monday, conventional wisdom on Wall Street has it that the iPhone 5 will outsell the iPhone 4S by a wide margin.

Among the handful of analysts who were willing to venture a guess, estimates for iPhone 5 sales this quarter (i.e. in the eight days from 9/21 and 9/29) ranged from 6 to 12 million. It took Apple three days to sell 4 million iPhone 4S units last year. Topeka’s Brian White estimates by day 3, the company will have sold 5 to 5.5 million iPhone 5s.

Stephen Baker begs to differ.

“The iPhone 5,” he writes, “will launch into a U.S. smartphone market with very different dynamics than the launch window of the iPhone 4 or the iPhone 4s.  And these dynamics have nothing to do with the strength of the new product offering, its hardware configuration, its marketing, or even its competition.  The U.S. smartphone market appears to be an increasingly mature one. A recent press release from NPD detailed that total growth in the US market was just 9 percent in the second quarter (Q2), and all that growth was the result of the increased prevalence of pre-paid devices in the market…

“As the market matures the challenges of growing faster than the industry multiplies. Blowing away all weakened competition, as Apple has done to this point, makes it infinitely harder to continue to blow away the remainder, because they are, by virtue of their current position, much more competitive than those that have fallen by the wayside.”

Baker may be right. But it’s also true that the NPD group measures only U.S. retail sales, while Apple sells all over the world. In fact, of the 37 million iPhones it sold in the December quarter last year — the first that included the iPhone 4S — nearly half (17 million) were purchased outside the U.S.

All this suggests that the number to watch for at Apple’s special event tomorrow is how many countries will be getting the new phone on Day 1. Last year, the iPhone 4S launched  initially in the U.S., Australia, Canada, France, Germany, Japan and the U.K..

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