"The iPad is not a computer, and competitors who approach it like the PC market will fail," says Apple's Steve Jobs. "Our competitors are looking at this like it's the next PC market."

"These are post-PC devices that need to be easier to use than a PC, more intuitive," said Jobs.

As always, it is somewhat difficult to separate marketing spin from analysis. On one hand, tablets are decidedly more useful as "content consumption" devices than as "content creation" devices. To the extent it can be argued that tablets are a new product category, they are "not PCs." That probably is most true at the moment for consumer users, who tend to own both PCs and tablets. 

On the other hand, business users seem to be rapidly adopting tablets as well, in ways that displace use of PCs. So is a tablet, in a business or consumer context, a new form factor for a PC, as the notebook was a different form factor, compared to desktop PCs? Right now, it seems hard to separate the marketing message from the use case. What seems to be obvious is that lots of people do not need much more than Web browsing and e-mail access from a computing device, much of the time. 

In part, that is because virtually every tablet user also has access to a PC when some features or applications are required. Early analysis often suggested that tablets would displace netbooks, however, one wishes to classify tablet devices. Early behavior suggests that is largely correct. Business users carry them to meetings and on trips where they would have carried a notebook in the past, or in cases where they might have carried only a smartphone. 

That alone suggests a shift in user behavior that is now obvious because there is a device implementation that makes the behavior clear. As the Internet has become more central, the need for all sorts of functions related to "printing things," "writing things" and "creating spreadsheets or presentations" has become something relegated to a desktop. 

On-the-go access to Internet applications and e-mail clearly has driven smartphone adoption, as it seems to be driving tablet adoption. 

Irrespective of how one defines "a tablet," Gartner recently has lowered its PC unit forecast for 2011 and 2012, based on expectations of weaker demand for mobile consumer PCs. Worldwide PC shipments are forecast to reach 387.8 million units in 2011, a 10.5 percent increase from 2010, according to Gartner's preliminary forecast. This is down from Gartner's previous projection of 15.9 percent growth this year.

Gartner expects worldwide PC shipments to total 440.6 million units in 2012, a 13.6 percent increase from 2011. This is down from Gartner's previous outlook of 14.8 percent growth for 2012. However one wishes to spin it, tablets get used in ways that are "post-PC" just as MP-3 devices or smartphones get used in ways that arguably are "post-phone" or "post-CD player."

“These are post-PC devices that need to be even easier to use than the PC, and even more intuitive,” Jobs said. The iPad is showing that people are ready for Web appliances in a big way.

Gartner analysts said that consumer mobile PCs have been the dynamic growth engine of the PC market over the past five years, averaging annual rates of growth approaching 40 percent. For much of this period, mobile PCs remained consumers' platform of choice for bringing the Internet into their daily lives. However, Internet access is now available through a multitude of mobile devices that allow consumers to engage in virtually all their favorite online activities without the need of a mobile PC.

"We expect growing consumer enthusiasm for mobile PC alternatives, such as the iPad and other media tablets, to dramatically slow home mobile PC sales, especially in mature markets," said George Shiffler, research director at Gartner. "We once thought that mobile PC growth would continue to be sustained by consumers buying second and third mobile PCs as personal devices."

"However, we now believe that consumers are not only likely to forgo additional mobile PC buys but are also likely to extend the lifetimes of the mobile PCs they retain as they adopt media tablets and other mobile PC alternatives as their primary mobile device," said Shiffler. "Overall, we now expect home mobile PCs to average less than 10 percent annual growth in mature markets from 2011 through 2015."