Steve Jobs would have you believe that the post-PC era is here. Market researcher Gartner seems to agree.

Apple iPad 2

At the rollout of the iPad 2 yesterday, the Apple CEO's carefully calculated musings about the post-PC world were meant to sow just enough doubt in consumers' minds that they think twice when buying that second PC. (And he was also careful to couch this as the post-PC era, not the post-Mac era. You can have your cake and eat it too if your Apple apparently.)

Maybe Gartner sees this as a self-fulfilling prophesy. Whatever the case, the market research firm made some dark prognostications today about the future of PCs. It seems that tablets will not be "additive"--a word that Intel likes to use--but subtractive. In short, instead of opting for that second PC and then maybe a tablet on top of that, consumers will opt for just the tablet.

"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," George Shiffler, research director at Gartner, said in a research note today. "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."

That last sentence is powerful. A consumer will hold off on buying a new PC and allocate that money instead to a tablet or other mobile device--like a high-end smartphone.

And the verdict "Overall, we now expect home mobile PCs to average less than 10 percent annual growth in mature markets from 2011 through 2015."

Overall, Gartner is lowering its PC unit forecast for 2011 and 2012. 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. But this is down from Gartner's previous projection of 15.9 percent growth this year. And Gartner expects worldwide PC shipments to total 440.6 million units in 2012, a 13.6 percent increase from 2011. This is also down from Gartner's previous outlook of 14.8 percent growth for 2012.

There were other factors too, like China. "These results reflect marked reductions in expected near-term unit growth based on expectations of weaker consumer mobile PC demand, in no small part because of the near-term weakness expected in China's mobile PC market, but also because of a general loss in consumer enthusiasm for mobile PCs," said Ranjit Atwal, another Gartner analyst.

But Gartner devoted most of the note to tablets. In a section titled "PCs' Limitations Are Exposed," Gartner said that "not too long ago, PCs were a 'fashion accessory' in mature markets with vendors linking themselves to fashion designers and even creating PCs specifically for women. The current 'cool' device is the smartphone, and now PCs will soon have to do battle with media tablets when they are launched in large numbers in the second quarter of 2011."

Mobile PCs are not keeping up with the times, according to Gartner. "Mainstream mobile PCs have not shed sufficient weight, and do not offer the all-day battery life, to substantiate their promise of real mobility. These limitations have become all the more apparent with the rapid spread of social networking, which thrives on constant and immediate connections. In short, all-day untethered computing has yet to materialize, and that has exposed the 'mobile' PC as merely a transportable PC at best," according to the note.