They may differ in the detail but multiple analyst firms are tipping that Mac sales for the quarter ending September 2009 rose faster than the industry average as well as showing useful year-on-year growth.

Gartner estimates that Apple shipped 1,572,000 Macs in the US alone, taking an 8.8% market share. This represents 6.8% unit growth from the year-ago quarter.

IDC's US figure is slightly higher at 1,640,000 Macs, and that estimate gives Apple 9.4% of the market with 11.8% year-on-year growth.

Either way, it's substantially better than the 3.9% (Gartner) and 2.5% (IDG) estimates of total PC market growth.

Still, any growth is welcome given the state of the economy during the last year or so. IDC previously recorded global unit sales declines of 6.8% and 2.4% in the first and second quarters of 2009. Apple returned to growth in the second quarter.

According to Computerworld, IDC vice president of clients and displays Bob O'Donnell said "Their [Apple's] sales just seem to defy all logic."

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Mac sales have remained strong amid a flight to the low end in the rest of the PC market. IDC's US estimates show Acer's unit sales grew 48.3% year on year (the company is primarily known for its netbooks), and notebook vendor Toshiba also grew strongly. The firm notes that most of the global growth comes from portable models, and that "Mini Notebooks are still making a substantial contribution."

Unless you count the MacBook Air as a netbook due to its light weight and thinness (most observers wouldn't, as its 13in screen and relatively powerful processor  - not to mention it's hefty price tag - take it out of the category), Apple does not have a product in the latter category.

However, its notebooks have proved consistently popular, especially in the third quarter, which includes the US 'back to school' buying season.

Neither IDC nor Gartner puts Apple in the top five vendors globally. Both put HP at the top of the pile with around 20% of the market, followed by Acer (14%), Dell (13%), Lenovo (9%), and Toshiba (between 5 and 8%).

And both show Dell as the only top five vendor with reduced unit sales during the quarter. If the analysts' preliminary figures are correct, the pioneer of low-cost PCs saw volume fall by 6.7% (Gartner) or 8.4% (IDC).

Although neither Gartner nor IDC has published global estimates of Mac shipments, figures are available from other sources. AppleInsider has conveniently summarised a pair of market analysts' estimates of global Mac shipments.

Piper Jaffray senior research analyst at Piper Jaffrey tips 2.8 million, RBC Capital Markets analyst Mike Abramsky says 2.9 million. Apple had a record quarter ending September 2008, when it shipped 2.6 million units.

The company will announce its quarterly results on Monday.