Have sales at iTunes picked up the pace in 2009? Yes, a bit. Are over-the-air downloads from iPhones the cause for the recent burst in sales volume? Doubtful.

Apple's CFO points to over-the-air capabilities as the reason behind the doubling of annualized iTunes sales, to four million per year from two million, in a post referenced at Digital Music News. In last week's Q3 earnings call, Apple announced it had broken the eight billion track mark in the prior week. In January of this year it announced it passed six billion track downloads. (The iTunes Wikipedia page tracks the store's progress and milestones reached.)

If torrid growth currently exists, it’s not happening in the U.S., the biggest recorded music market in the world and (back of the envelop calculation coming) worth about 45% of iTunes’ unit sales since inception. According to weekly sales data from Nielsen SoundScan, total track sales have been more or less flat since the standard post-Christmas, gift card-fueled peak. To arrive at figures for tracks derived from album sales, Billboard counted each digital album sale as 13 track sales. (Billboard uses ten tracks per album for Track Equivalent Album calculations, which is better for financial comparisons.) If the iPhone was driving an increase in sales, there would be some evidence in the weekly numbers.

A few possible scenarios to explain why iTunes sales could be flat in recent months:

-- If iPhone over-the-air downloads are occurring on a meaningful level, it’s possible they are downloads that would have taken place anyway. Meaning the iPhone has not resulted in any greater number of impulse purchases than iTunes accessed through a computer. Sales are merely shifted from one piece of hardware to another.

-- It's possible digital sales would have declined since February had it not been for impulse, incremental purchases. In that case, the iPhone is actually helping keep sales from declining.

-- It's also possible, although unlikely, that sales at all other stores have decreased and iTunes has been capturing a larger share of download purchases.

Based on the shape of the sales curve and general trends in the marketplace, it appears iTunes sales are on auto-pilot and growth is not being driven by over-the-air downloads. Total sales spiked around Christmas, leveled off by February and dropped when some track prices rose to $1.29. That’s what would have happened with or without the success of the iPhone and iPod touch.