A pair of reports from Piper Jaffray’s Gene Munster tell the story.

The first, issued early Monday morning, gave the results of a headcount performed at three Apple (AAPL) retail stores on Black Friday, the traditional start of the holiday selling season. Although the stores were busy, his team counted an average of 8.3 Mac sales per hour, down 36% from the 13 Macs per hour they observed on the same day last year.

Munster’s second note, sent nearly seven hours later, reported on comScore data indicating that sales at traffic on Apple’s online store Friday was up 39% year over year.

“Apple’s online store had a big day on Black Friday,” Munster concludes, “offsetting the y/y decline in our retail store checks.”

Based on NPD data that showed U.S. Mac sales up 7% year over year in October, Munster had previously estimated that Apple would sell 2.856 million Macs in the quarter that ends Dec. 26. That’s up from 2.524 million Macs in the same quarter last year, but down from the record 3.053 million Apple sold last quarter.

UPDATE: More field checks and estimates below the fold from Kaufman Bros.’ Shaw Wu, Thomas Weisel’s Doug Reid and Deutsche Bank’s Chris Whitmore.

[Follow Philip Elmer-DeWitt on Twitter @philiped]

These just in:

• Wu writes: “Our field checks indicate strong foot traffic at Apple stores and that Macs, iPhones and iPods saw strong uptake … We believe our forecast looking for 2.9 million Macs is likely conservative. Currently, consensus estimates look for 2.85 million Macs.”
• Reid’s team did store checks at 41 authorized retail locations in the U.S., Canada and the U.K. and came away with “increased confidence” in his estimate of 3.06 million Mac sales this quarter, up 21% year over year.
• Whitmore’s team got the shoe-leather prize, conducting store checks over Black Friday weekend at more than 120 Apple retail stores and 25-plus AT&T outlets. He concludes that both Macs and iPhones are tracking “in-line to above” his unit estimates (8.5 million iPhones and 3 million Macs) while iPods are tracking “roughly in-line” with his estimate of 23 million.

CORRECTION: Several news sites — including this one — repeated Munster’s assertion that Apple’s online sales on Black Friday increased 39% year over year. But comScore’s report clearly states that it was measuring Apple.com’s unique visitors, not its online sales. See here.