Long relegated to the status of a niche player catering to Mac fans, Apple now leads all U.S. personal computer manufacturers in terms of market share—if the iPad is considered a PC.

Deutsche Bank analyst Chris Whitmore compiled data that shows Apple owns an industry leading, 25% share of the U.S. PC market when iPad sales added to Mac sales. Whitmore's figures show Apple ahead of Dell and HP, each of which holds a share of about 20% as of the end of the third quarter, and Acer, with a 9% share.

Excluding iPad sales, Apple's share of the U.S. personal computer market is just 9%. The iPad "is driving a rapid, unprecedented shift in the structure of the computing industry," said Whitmore, in a research note.

The question, of course, is whether the iPad is really a PC. If consumers consider Apple's tablet to be a wholly new type of gadget, iPad sales shouldn't have much of an impact on the traditional computer market. But if shoppers view the iPad as a PC in its own right or, at least, a PC replacement, then iPad sales will likely take a bite out of PC maker's revenues, not to mention Microsoft's sales of Windows 7.

There's some evidence that the latter scenario is beginning to play out. Research group NPD recently released a survey that found 13% of iPad buyers would have purchased a PC had the iPad not been available. The iPad and Android-powered tablets are expected to be the hottest selling products of the upcoming holiday season.

Furthering blurring the lines, Apple recently struck deals with mass merchandisers Wal-Mart and Target, under which the department stores are stocking the iPad. Traditionally, Apple products have been reserved for the company's own stores, or specialty electronics retailers like Best Buy.

The iPad has become one of the fastest selling gadgets in tech industry history since Apple introduced it April 3rd. Apple sold 300,000 of the devices the first day they were on sale, and more than 2 million units in the first 80 days.

Apple is slated to report earnings Monday after the close of trading, at which time it's expected to divulge more data about iPad sales.

This year's InformationWeek 500 companies are refocusing their IT efforts on innovation and growth, often after deep cuts during the downturn. Find all charts and stats on the top 500 IT organizations in the nation. Download the report here (registration required).