Apple is usually considered an also-ran in the global PC market.

But the chart at right, taken from a note to clients issued Friday by Deutsche Bank’s Chris Whitmore, tells a very different story.

Whitmore believes that investors will increasing include tablet computers when assessing market share trends, and this diagram illustrates what that might look like.

Starting with IDC’s global portable computing market share survey for the June 2010 quarter, he has redrawn Apple’s (AAPL) share, adding the 3.27 million iPads the company sold in the quarter to its 2.47 million MacBooks and MacBook Pros.

“When including the iPad as part of the NB [notebook] market,” he writes, “Apple leapt over Asus, Lenovo, Toshiba and Dell in terms of global unit share.”

Viewed this way, Apple goes from being No. 7 in the worldwide portable computer market to No. 3, after only Hewlett Packard (HPQ) and Acer. And it’s not stopping there, says Whitmore.

“Our retail checks suggest this share shift continues in July,” he writes, “where the iPad is directly cannibalizing demand for other vendors’ NB products. Remarkably, Apple’s traditional MacBook business posted accelerated unit growth on a Y/Y basis in 2Q despite the launch of the iPad while every other Top 5 vendor slowed.”

The effect is even more striking when Whitmore graphs IDC’s results in terms of market share growth. Even without the iPad, Apple was moving away from the rest of the pack. With the iPad, it’s nearly off the charts. See below.

Another way of looking at the data:

[Follow Philip Elmer-DeWitt on Twitter @philiped]