Apple (AAPL) computers sold surprisingly well in a shaky economy last quarter, according to a pair of preliminary reports issued Wednesday by Gartner and IDC.

The big winner this summer was Acer, which became the world’s undisputed No. 2 computer maker, after Hewlett-Packard (HPQ), on the strength of netbooks that sell for under $400.

But Mac sales also grew, despite an average selling price of just over $1,200 and profit margins that are the envy of the industry.

Reporting only domestic sales, Gartner estimates that Apple shipped 1.572 million Macs in the United States last quarter, up 6.8% year to year, for an 8.8% market share.

IDC’s numbers were slightly higher: 1.64 million Macs, up 11.8%, 9.4% market share.

These seem to be record numbers. Twelve months ago, Gartner put Apple’s U.S. market share at 9.5% and IDC had it at 9.1%, but those were preliminary reports. Both firms revised their 2008 Q3 Mac numbers sharply downward and now say Apple’s share of the U.S. market last year was actually 8.6%.

Neither Gartner nor IDC reports Apple’s worldwide Mac sales. For that we’ll have to wait until Monday, Oct. 19, when Apple issues its quarterly earnings report. Analysts we surveyed put total Mac sales anywhere between 2.6 million and 2.9 million, with a consensus of 2.76 million.

UPDATE: Reader SMR in Nashua, N.H., points out that if you go back far enough, you can find years in which Apple enjoyed a larger share of the U.S. PC market. In 1994, for example, it was No. 2 (after Packard Bell) with a 14.7% share. In 1981, it had 17%, which put it in the No. 2 spot after Radio Shack (20%). Source: Ken Paulson’s Chronology of Apple Computer Personal Computers.

Below the fold: Gartner and IDC’s current U.S. and worldwide PC numbers:

Gartner:

IDC: