After launching two new iPhone models in late September, Apple has now launched two new iPad models, as well. But after a quieter-than-expected iPad Air launch on November 1, Apple today suddenly made the second new iPad model, the iPad mini with Retina display, available to customers online and in retail stores.

Although the iPad Air launch date was pre-announced by Apple, the firm's retail stores didn't experience the long lines of eager customers that have typified such launches in recent years. The iPad mini with Retina display, however, could be a lot harder to get, especially if you don't move quickly. Apple has hinted that its high-resolution display is in short supply. So today's opening of sales was a surprise.

"We're excited for customers to experience the new iPad mini with Retina display," Apple Senior Vice President Phil Schiller said in a prepared statement. "We're working hard to get as many as we can in the hands of our customers."

As is its custom, Apple is also selling its previous-generation, non-Retina iPad mini version as an entry-level product. So it now offers a wide variety of iPad models, including the base iPad mini ($299 to $429 for 16GB of storage, in gray and white versions, with or without cellular data), the iPad mini with Retina display ($399 to $829, in many versions with 16GB to 128GB of storage), the 2011-era iPad 2 ($399 to $529 for 16GB of storage, in gray and white versions, with or without cellular data), and the iPad Air ($499 to $929, in many versions with 16GB to 128GB of storage).

The iPad mini with Retina display is interesting because it ties a very high-resolution screen—a market-best 2048 x 1536 resolution—to a highly mobile 8" screen. That said, the device's pixel count, 326 per inch, is essentially identical to that of some of the competition: The 7" 1920 x 1200 screens used by the Amazon Kindle Fire HDX and Google Nexus 7 both provide 323 pixels per inch. And those devices cost much, much less than the iPad Air with Retina display. Both start at $229, a full $170 less than a comparable iPad mini.

This price differential, combined with the steadily improving competition—the Kindle Fire HDX and Google Nexus 7 are both excellent, high-quality devices—and the burgeoning ranks of these competitors, has triggered a situation that mimics what happened to Apple previously in the PC and smartphone markets. Apple, which once dominated the modern tablet market it created in 2010, has fallen behind.

According to IDC, although Apple's iPad lineup is the number-one-selling tablet family overall, the devices have fallen further and further behind the Android competition in each quarter. In Q3 2013, the iPad fell to 29.6 percent market share from 40 percent in the same quarter a year ago, with Android taking up most of the rest. More alarming, all of Apple's competitors have seen massive growth in this time frame—Samsung jumped 123 percent, ASUS is up 54 percent, Lenovo roared a full 420 percent, and Acer leaped 346 percent—while Apple's year-over-year growth was essentially flat.

Apple defenders will point out, fairly, that their favorite company continues to make the bulk of the profits in this market thanks to the company's high prices and heady margins. And because of its annual product launch schedule, Apple could see a temporary reversal of its market share decline this quarter, as it does each year during its iPhone launch quarter. But the company is unlikely to lower prices to grab more share over the long term, preferring to keep this healthy business as profitable as possible as it plots new markets to exploit in the years ahead.

I've reviewed the Google Nexus 7, Amazon Kindle HDX, and a Windows mini-tablet entry, the Dell Venue 8 Pro, this year, and will review the iPad mini with Retina display soon. Stay tuned to the SuperSite for Windows for more information.