The Cupertino-based company saw Mac sales rise more than 38 percent to nearly 1.4 million units in the US during the three-month period ending June, making it the country's third largest PC manufacturer behind Dell and HP, who saw sales rise 11.9 percent and 5.6 percent to 5.25 million and 4.167 million units, respectively.

Overall, Apple's share of the US PC market was up more than 2 percent compared to the same period last year. Total US PC shipments reached 16.5 million units in the second quarter of 2008, a 4.2 percent increase from the same period last year.

"Dell continued to be the market leader with PC shipments accounting for 31.9 percent of the U.S. market in the second quarter of 2008," said Mika Kitagawa, principal analyst for Gartner's Client Computing Markets group. "Apple's PC shipments grew 38.1 percent in the quarter. The home PC segment continued to be the strongest driver for Apple, as well as sales into the education segment."

Apple edged out Acer, who saw sales decline more than 20 percent, by a little over 65,000 units. Meanwhile, Toshiba saw its own PC sales rise just 2.8 percent to 907,000 units, good enough for a 5.5 percent share and fifth place.

Overall, global PC shipments reached 71.9 million units in the second quarter of 2008, a 16 percent increase year-over-year. Apple, however, did not place within the top five manufacturers worldwide, meaning its global share of the market remains behind the 4.4 percent achieved by fifth place holder Toshiba, which sold a total of 3.14 million units in the second quarter.

"Mobile PCs continued to lead unit growth across all regions as the average selling price (ASP) of mobile PCs declined sharply relative to desk-based PC ASPs," Kitagawa said. "Economic uncertainties have hit PC revenues, resulting in steep ASP declines, especially in markets such as the United States and the Europe, Middle East, and Africa (EMEA) region."

Preliminary Worldwide PC Vendor Unit Shipment Estimates for 2Q08 (Thousands of Units) | Source: Gartner

Holding on to its No.1 worldwide ranking was HP, which shipped just over 13 million systems, representing 17.1 percent growth and a 18.1 percent share. Dell followed with sales of more than 11.2 million systems for a 15.6 percent global share, representing 21.9 percent yearly growth. Acer (9.4 percent share), Lenovo (7.8 percent share), and Toshiba (4.4 percent share) rounded out the top five.

Slightly more cautious than its research rival, IDC said Wednesday in its own second-quarter study that Apple has finally broken through a symbolic barrier and will likely wind up tied for the third spot among U.S. computer vendors.

The Mac maker is said in the IDC prediction to have shipped just short of 1.33 million computers between April and June, which would give it the same 7.8 percent estimated American market share as Taiwan-based Acer.

Apple's surge signals a year-over-year growth in sheer units of about 31.7 percent, continuing a relatively rapid sales acceleration that began roughly during the company's switch to Intel processors.

Acer technically grew faster at 49.9 percent but was primarily helped by its merger with Gateway in the past year; if the combined share of the two is measured from last year, the unified entity actually shrunk by 28.1 percent, according to IDC.

Apple remains unlikely to challenge the top two vendors in the country, Dell and Hewlett-Packard, any time soon, as the two alone account for more than half of all US sales in the most recent results at 32 percent and 25.1 percent of the market respectively. The two are nonetheless growing relatively slowly at 12.1 percent year over year for Dell and just 5.9 percent for HP.

Toshiba lost share over the period, dipping from 5.4 percent to 5.2 percent.

Preliminary U.S. PC Vendor Unit Shipment Estimates for 2Q08 (Thousands of Units) Source: IDC