You might think that estimating iPad sales for the fiscal quarter that ended on June 26 would be a pretty easy call for analysts, given that Apple (AAPL) let it be known five days before the end of the quarter that it sold its 3 millionth iPad on June 21.

Yet the analysts we polled offered estimates that ranged from a high of 3.75 million to a low of 2.5 million. In fact, the most recent published estimates of five analysts are still at 3 million or fewer. (See below the fold.)

To be fair, everybody was surprised by how quickly iPad sales took off, including Steve Jobs. In a long note to clients published Wednesday, Bernstein Research’s Toni Sacconaghi pointed out that the “scorching pace” of iPad sales put it ahead of the iPhone, iPod Touch and all netbooks for first full quarter sales.

By the end of the quarter, iPads were selling at a rate of 45,000 a day. Extrapolating from the iPad’s current trajectory, Apple could sell from 25 million to 42 million units in fiscal 2011, its first full year of sales.

What would that mean for Apple’s bottom line? Sacconaghi does the math:

“Under a scenario by which Apple sells 25M iPads in FY 11,” he writes, “revenues would amount to about $15B, and likely grow about $10B [year over year]. This would make iPad revenues in their first full year nearly as large as Apple’s current Mac ($18B) business, and contribute nearly twice as much in [year over year] revenue growth than the iPhone.”

But Sacconaghi isn’t buying it. He thinks the pace will slow dramatically once the sales to the Apple enthusiasts are out of the way. His Q3 estimate, while over 3 million, is on the low end of our poll. And for fiscal 2011, he’s projecting sales of 18 million iPads, not 25 or 42 million.

Sacconaghi likes to position himself as an Apple skeptic. What he doesn’t mention in his July 7 report was that 3 months earlier he was predicting that Apple would sell only 6.82 million iPads in 2011.

Below: The Q3 estimates we’ve gathered so far. We’ll add more as they come in.

Analyst, affiliation	iPads (millions)	Date of estimate
Mark Moskowitz, J.P. Morgan	3.75	7/1/10
Tavis McCourt, Morgan Keegan	3.50	7/7/10
Shaw Wu, Kaufman Bros.	3.50	7/6/10
Andy Zaky, Bullish Cross	3.47	6/27/10
Dennis Hildebrand, Apple’s Gold	3.45	7/7/10
Turley Muller, Financial Alchemist	3.38	7/7/10
Robert Paul Leitao, Apple Finance Board	3.30	7/2/10
Horace Dediu, Asymco	3.30	6/29/10
Yair Reiner, Oppenheimer	3.30	6/30/10
Brian Marshall, Gleacher & Co.	3.30	7/7/10
Daniel Tello, Deagol’s AAPL Model	3.26	6/30/10
Jeff Fosberg, Apple Finance Board	3.25	6/3/10
Alexis Cabot, Apple Finance Board	3.25	7/6/10
Ralph Schackart, William Blair	3.25	6/28/10
Gene Munster, Piper Jaffray	3.20	7/6/10
Toni Sacconaghi, Bernstein Research	3.20	7/7/10
Richard Gardner, Citigroup	3.10	6/28/10
Ashok Kumar, Rodman & Renshaw	3.00	7/1/10
Jeff Fidacaro, Susquehanna	2.82	6/18/10
Ben Reitzes, Barclay’s Capital	2.80	6/22/10
Kathryn Huberty, Morgan Stanley	2.75	5/25/10
Mike Abramsky, RBC Capital	2.50	5/20/10

. . .
See also:

• iPad sales accelerate
• How many iPhones did Apple sell?

[Follow Philip Elmer-DeWitt on Twitter @philiped]