The tablet category expanded its role in the mobile computer market, with shipments of 72.7 million units, accounting for 25.5% of mobile computer shipments in 2011, according to the latest date from NPD DisplaySearch (http://www.displaysearch.com).

The year-over-year shipment growth rate for tablets such as the iPad in 2011 is expected to reach 256%, as strong tablet shipments in the third quarter of 2011 appeared to be a prelude to equally vigorous demand in the fourth quarter, notes the research group.

Introductions of significantly competitive new devices in the fourth quarter, strong demand for incumbents, and additional distribution channels have been catalysts for accelerated tablet shipment growth, according to NPD DisplaySearch. Tablet shipment growth is happening as the research group has slightly lowered its notebook shipment forecast.

Notebook PC shipments in 2011 are now expected to reach 187.5 million units, up 12% year-over-year, but slightly less than the previous forecast of 188 million. Mini-notebook shipments are expected to reach 25.2 million units, up 20% year-over-year. Overall mobile year-over-year shipments in 2011 are projected to reach 285.4 million units, up 31% year-over-year.

“In the short term, tablet computers will be the growth accelerant in the overall mobile computer market, as macroeconomic factors and hard drive motor supply issues impact the market for notebook computers,” says Richard Shim, senior analyst at NPD DisplaySearch. “In the longer term, notebook PC shipments will bounce back as ASPs continue to decline, Windows 8 launches, and new form factors, such as ultrabooks, continue to emerge.”

By 2017, notebook shipments are forecast to reach 432 million units, and tablet PC shipments are projected to reach 383.3 million units. Ultrabooks are an emerging form factor that is expected to contribute to demand in later years of the forecast, according to DisplaySearch. Demand for ultrabooks will be driven by consumer interest in sleek design and convenience like instant-on and long battery life, says the research group.

Tablet shipments will be driven by increasingly powerful multi-core processors, mature operating systems, growing application libraries, and higher resolution panels. NPD DisplaySearch forecasts that tablets with 250 to 300 pixel per inch panels will make up nearly 25% of shipments in 2012.