“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.”

That’s George Shiffler, research director at Gartner, Inc., giving his clients the bad news that the 15.9% growth his company had projected for notebook computer sales in 2011 is looking more like 10.5%.

That’s a real setback for all the major PC manufacturers. Except, perhaps, Apple (AAPL).

“We once thought,” Shiffler continues, “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.”

Mainstream mobile PCs — read: Microsoft (MSFT) Windows laptops — are no longer the “cool” accessory, according to Gartner. They have not shed sufficient weight and they haven’t achieved all-day battery life.

“These limitations,” the report concludes, “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.”

You can read the full Gartner press release here.

Also on Fortune.com:

• Steve Jobs’ post-PC credo
• Apple posts iPad 2 keynote video
• iPad 2: The view from Brooklyn

[Follow Philip Elmer-DeWitt on Twitter @philiped]