iPod unit sales, despite seasonal spikes, have been drifting down since Christmas 2008, when they peaked at 22.7 million for the quarter (Apple’s fiscal Q1 2009).

That quarter also represented the low point in iPod average selling price (ASP) per unit, $148. That’s because Apple’s (AAPL) business model was shifting from selling dedicated music players to selling more expensive hand-held computers, like the iPhone and the iPod touch, that also play music. Apple reports iPhone sales separately, but you can trace the growth of iPod touch sales in the chart above, and in the iPod’s rising ASP, which hit $176 in fiscal Q1 2011.

All this is by way of explaining an anomaly that surfaced when we polled 43 analysts — professionals and amateurs — for their iPod unit sales estimates for fiscal Q2 2011, the quarter that ended two weeks ago.

In every other category — from top to bottom line — the amateurs’ estimates are higher than the professionals’. But when it comes to iPod unit sales, the pros come in, on average, 1.66% higher. For once, the high estimate (11.32 million iPods) was offered by a professional, Gabelli’s Hendi Susanto, and the low (8.5 million) by an amateur, Luke Kittell from The Mac Observer’s Apple Finance Board.

It seems the amateurs, as a group, are betting that Apple will have sold fewer music players but more hand-held computers.

Below: The estimates for Q2, along with each analyst’s ranking in the Q1 2011 earnings smackdown. We’ll find out who was closest to the mark this quarter when Apple reports its earnings on April 20.

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Also on Fortune.com:

• How many Macs did Apple sell last quarter?
• How many iPhones did Apple sell last quarter?
• How many iPads did Apple sell?

[Follow Philip Elmer-DeWitt on Twitter @philiped]