Low-end mobile PCs are expected to drive worldwide market growth in the double digits through 2011, a market research firm said Wednesday.

PC shipments will grow by 15.7% this year to 311 million units, IDC said in its latest forecast. Growth is expected to remain in the double digits, falling to 9% in 2012, when shipments are expected to top 482 million.

The popularity of low-cost notebooks among consumers looking to buy a PC for the first time or as an additional system will boost shipments and push average prices down, IDC said. Ongoing wireless infrastructure upgrades and the burgeoning role of portable PCs as part of a "digital lifestyle," rather than just computing devices, also will sustain growth in both consumer and commercial segments.

"We continue to see a rapid transition to portable PCs around the world, even as economic pressures rise," IDC analyst Loren Loverde said in a statement. "The trend reflects the increasing importance of computing, not just in the home or office, but as an integrated part of our lives."

Whereas consumer PC success was once measured by the number of computers per household, that metric has shifted to the number of machines per person. "The increasing form factor diversity in notebooks as well as desktops is enabling people to justify multiple PC purchases," IDC analyst Bob O'Donnell said.

In the United States, shipments of portable PCs are expected to increase by 17.7% this year, 16.3% in 2009, 15.6% in 2010, 13.9% in 2011, and 12.9% in 2012, when shipments are expected to reach 61.1 million units, according to IDC. Shipments this year are projected to reach 35.3 million units.

Mobile PC shipments outside the United States are expected to increase at a higher rate, growing by 44.7% this year, 29.6% next year, 23.3% in 2010, 15.9% in 2011, and 13.5% in 2012, when shipments are forecast to reach 237.2 million units from 112.8 million this year.