By Philip Elmer-DeWitt
July 21, 2013

FORTUNE — Analysts tend to think of the iTunes store as a metaphor. It’s a “moat” that protects Apple (AAPL) from competitors. Or something “sticky” — like honey or flypaper — that keeps fickle customers from flying away.

But since Apple reclassified its revenues in January and consolidated iTunes, Software and Services into one line item, it’s become clear that iTunes is also a large and fast-growing business in its own right, one that generated $12.9 billion in fiscal 2012.

If iTunes had been listed separately in the current Fortune 500, it would have come in at No. 218, ahead of Texas Instruments, Nordstrom and Con Ed.

iTunes, Software, Services was the company’s second fastest growing revenue stream in fiscal Q2. When Apple reports its earnings on Tuesday, according to Fortune‘s panel of Apple analysts, it will have passed both the iPhone and the iPad to take first place in terms of year-over-year growth.

iTunes estimates were optional in our quarterly polling, but so far we’ve received numbers from 40 analysts — 21 Wall Street professionals and 19 amateurs.

Their estimates ranged from the high of $5.35 billion submitted by Baird’s William Power to a low of $3.3 billion from Sunil Shah, an independent from the Braeburn Group.

The average estimate — $4.1 billion — is 28% higher than the $3.20 billion listed for the same quarter last year in Apple’s “reclassified summary spreadsheet.”

In a reversal of their usual roles, the Street is more bullish about iTunes this quarter than the amateurs. The pros are calling for 31% growth; the amateurs for 25%.

We’ll find out who was closest to the mark when Apple reports its earnings for fiscal Q3 2013 after the markets close on Tuesday, July 23.

Below: The individual analysts estimates, with the pros in blue and the indies in green. Thanks once again to Posts at Eventide‘s Robert Paul Leitao for pulling together the Braeburn Group numbers.

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