Worldwide sales of portable computers is expected to grow 26% to 215 million units according to a new report by DisplaySearch.

The research firm is also predicting a big increase in the size of the slate-style PC market which will be driven by the Apple iPad and slates from other PC brands that are expected to be launched this year.

DisplaySearch expects slates to take some market share from clamshell-style netbook PCs, as well as lure customers that desire more functionality away from the e-reader market. The company also foresees the majority of slate volume to ship into the North American and Western European markets due, in large part, to Apple’s distribution plans and known content agreements.

The mini-note/netbook/slate category will be the fastest-growing segment of the portable PC market for 2010, propelled in part by the introduction of slates. However, clamshell style devices are also predicted to grow by 25% thanks to a recovery in corporate spending in the second half of the year.

Due to the lower average selling prices of netbooks and slate style computers, overall portable PC revenues are expected to grow just over 6.4% to $117 Billion U.S. , about where they were in 2008.