Wall Street was more than a little skeptical in July when Apple (AAPL) CFO Peter Oppenheimer offered his revenue guidance for the quarter that ended two weeks ago: $18 billion — a 47% increase from Sept. 2009.

“We hadn’t girded ourselves for a mammoth revenue forecast,” wrote Oppenheimer’s Yair Reiner the next day, “and neither, we believe, had the Street.”

Like most analysts who follow Apple, Reiner assumed that Oppenheimer had abandoned his usual practice of low-balling the company’s forward-looking statements and was trying to blow a little smoke up the Street’s skirt. In the previous three quarters, Apple had beaten its own revenue guidance by an average of 19.3%. That would suggest actual Q4 revenues of nearly $21.5 billion — a number Financial Alchemist‘s Turley Muller described at the time as “too mind-blowing to even contemplate.”

A week after Oppenheimer’s report, the Street’s Q4 revenue consensus, according to Thomson Financial, was $18.29 billion — a measly 1.6% beat. See Who buys Apple’s $18+ billion quarter?

Three months and a few million iPhones and iPads later, most Apple watchers have changed their tune.

Of the nine blogger-analysts we polled, all but two expect Apple to report earnings of $20 billion or more, including Muller who’s put his marker on $20.313 billion. Among the 21 professional analysts polled, only one –Rodman & Renshaw’s Ashok Kumar — has crossed that divide. On Thursday, Reiner hiked his estimate to $19.864 billion. The Street’s consensus is $18.76 billion.

In the chart above, we’ve combined the pros and the bloggers to get an average estimate of $19.34 billion.

Below: The revenue and earnings-per-share numbers we’ve gathered so far. We’ll find out whose were closest after the closing bell on Monday Oct. 18th.

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[Follow Philip Elmer-DeWitt on Twitter @philiped]