What if the iPad were a PC?

If you throw tablets in the mix, Apple just became the U.S.'s No. 1 computer maker

Click to enlarge.

"The iPad," writes Deutsche Bank's Chris Whitmore in a note to clients issued Monday, "is driving a rapid, unprecedented shift in the structure of the computing industry."

To illustrate that point, Whitmore has taken a chart of domestic personal computer market shares over the past seven quarters as measured by IDC, which doesn't consider tablets to be personal computers, and redrawn it with the iPad added in (see right).

Viewed this way, things suddenly looks very different. As Whitmore points out:

• The U.S. PC market grew an anemic 4% year-over-year in calendar Q3 without the iPad, according to IDC. Include the iPad, as Whitmore feels one ought to, and that growth jumps to a robust 24%.
• Exclude the iPad, and Apple's (AAPL) personal computer sales grew 24% year-over-year. Include them, and Apple's unit sales soared roughly 250%. By comparison, Hewlett-Packard (HPQ) grew 3%  year-over-year and Dell (DELL) units fell 5%.
• When the iPad is part of the mix, Apple's share of the U.S. PC market is about 25%. That makes it the market leader, having gained a remarkable 18 points in the space of two quarters.

NOTE: Several readers have suggested that Whitmore is comparing worldwide iPad unit sales with PC sales that are U.S. only. He replies that his iPad estimates are U.S. only. "It says so in the note," he says.

[Follow Philip Elmer-DeWitt on Twitter @philiped]