The iPad is Apple’s gateway to notebook PC dominance — if you agree with Deutsche Bank’s Chris Whitmore’s reasoning that investors will begin to look at tablets as notebooks. When viewed in that light, Apple is now the No. 3 notebook PC company — trouncing Dell, Asus, Lenovo, and Toshiba — reports Fortune.

Whitmore used IDC’s global portable computing market share survey for the last quarter, and added the 3.27 million iPads the company sold to its 2.47 million Macbooks and Macbook Pros. That brought the grand total of Apple’s portable computing devices to 5.74 million in the last quarter, and raised the company from N0. 7 in the global portable market to No. 3.

That growth isn’t stopping anytime soon, according to Whitmore:

Our retail checks suggest this share shift continues in July, 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.

Not surprisingly, the inclusion of the iPad shows some shocking growth in Apple’s portable computing market share (see Figure 3 below). Also interesting: Apple’s market growth based on Macbooks and Macbook Pros alone also grew, while all of its competitors slowed.

Whitmore’s calculations aren’t exactly complex, but given the growing importance of tablets, it may not be that far-fetched to include them among other portable computers. And if we’re adding the iPad to Apple’s mobile computing totals now, we’ll also be doing the same for competitors in the future as well.