The two companies projected that U.S. Mac sales during the quarter ending Sept. 30 were down from the same period the year before.

IDC estimated that Apple sold 2.06 million Macs, down 7% from the 2.21 million machines sold in the same quarter of 2011. Gartner pegged the decline at 6.1% on sales of 2.08 million Macs, compared to 2.21 million the year before.

If accurate, the sales decline would be the largest by Apple since the year-over-year. That quarter was in the midst of the U.S.'s worst economic crisis since the 1930s.

There's no guarantee that either IDC's or Gartner's estimate is on target -- IDC has called ballparking U.S. sales a "very difficult exercise" in the past -- or that Apple will post negative growth numbers for worldwide sales when it releases its earnings later this month.

As proof, Apple said second quarter sales in its "American" region -- which includes the U.S., Canada, Central America and South America -- was 1.52 million Macs, a number significantly lower than either estimate last July by IDC and Gartner, who had projected U.S.-only sales of 1.81 million and 1.91 million, respectively.