Mac Shipments Up. Also, Mac Shipments Down.

Domestic Mac shipments for the second quarter of 2009 rose to 1.422 million, a 2.5 percent year-over-year increase.

Or, they fell to 1.2 million, a decline of 12.4 percent.

All depends on whom you believe, Gartner or IDC. According to Gartner, Apple has an 8.7 percent share of the U.S. market. According to IDC, that share is 7.6 percent. In a quarter that saw overall PC shipments in the U.S. decline 1.2 percent or three percent, that’s either pretty damn good or troubling, again depending on whom you believe.

Good thing both firms describe these estimates as “preliminary,” right?

Anyway, according to Gartner’s preliminary data, Apple (AAPL) maintained its position as the nation’s fourth-largest computer maker in the second quarter, ranking between Acer and Toshiba. According to IDC, it dropped to fifth, one spot above the dreaded “All Other Vendors” segment.

So what are we to make of this? A disparity of more than 200,000 units between the the Q2 domestic Mac shipment estimates of two top market research outfits? Is it a 2.5 percent year-over-year increase. Or a 12.4 percent year-over-year decrease?

I have no idea. And clearly, Gartner (IT) and IDC (IDC) don’t either. Best to just wait until next Tuesday when Apple announces definitive sales figures during its third-quarter earnings.