The conventional wisdom — reflected in Arik Hesseldahl’s “Apple’s iPod Problem” in Businessweek — is that everybody who wants an iPod already has one, and doesn’t see much reason to upgrade, especially with the economy in the dumps. “As a result,” he wrote, “some analysts believe this will be the first quarter since the iPod was introduced in 2001 that sales will decline from the year-earlier quarter.”

Piper Jaffray’s Gene Munster is one of those analysts. He’s projecting sales of 18.6 million units in Apple’s December quarter, down 16% from 2008 Q1.

A cooling off of this magnitude would mean that Apple’ (AAPL) other business units — chiefly the Mac and the iPhone — need to pick up the slack if the company is to continue the quarterly growth to which investors have become accustomed.

But the reality on the ground, according to Kaufman Bros.’s Shaw Wu, is that iPods are selling like Christmas hotcakes.

In a note issued earlier this week, he reported that Amazon (AMZN), whose giant warehouses tend to have plenty of iPods in stock, was reporting a backlog of several models.

“iPod touch was surprisingly stocked out and now has lead times of 11 days for the 8 GB model and three to five weeks for 16 GB,” Wu wrote. “This is likely due to unexpected strong demand and we find this interesting as iPod touch is Apple’s highest-end iPod.” (link)

Now AppleInsider‘s Katie Marsal reports that Apple is “scurrying to restock some of its reseller channels” as those iPod shortages spread.

In a new report to clients issued Wednesday, Wu writes that spot checks are turning up shortages at Best Buy (BBY), Target (TGT), Wal-Mart (WMT) and Crutchfield.com.

“Frankly, we find these sell-outs on iPods surprising, given how difficult the macroeconomic environment is,” he told clients. “From our assessment, we believe iPod is holding up better than most, due to its relatively low [selling price] and strong consumer understanding of the value it provides.” (link)

Wu estimates that Apple will sell some 21 million iPods this quarter — less than the 22 million it sold the year before, but not by much.