In December, sales stemming from smartphones and tablets accounted for 11% of total online sales, up from 5.5% in December 2010, according to the IBM Benchmark report that tracks online sales of some 500 retailers. Total m-commerce sales for 2011 were expected to exceed $10 billion, according to the Internet Retailer Mobile Commerce Top 300.

The IBM Benchmark also found that 14.6% of all online sessions on retailers’ sites were initiated from a mobile device, more than double the rate of 5.6% during the same period in 2010. This includes traffic to m-commerce sites and the e-commerce sites of retailers without m-commerce sites.

When it comes to closing a sale, the iPad bested other mobile devices. The conversion rate for shoppers on iPads was 6.3% last month compared with 3.1% for mobile devices overall.

Total U.S. online sales grew 7.5% in December 2011 compared to December 2010, IBM finds. ComScore Inc. estimates November-December online sales increased 15% year over year; comScore bases its estimate on a consumer panel that includes some 1 million U.S. online shoppers.

Department stores continued to offer an array of compelling deals and promotions that caught the attention of consumers, IBM says. Department store online sales were up 18% over December 2010, the benchmark report finds. Apparel sales were up 16.3%; health and beauty 16%; and home goods 15.6%.

“This past December consumers remained committed to finding the best online deals whether through their PC or mobile device,” says John Squire, chief strategy officer for IBM Smarter Commerce. “By employing a smarter approach to commerce, many retailers were successful in helping to connect their customers with the best deals from anywhere and at any time, even on Christmas Day where online shopping grew by 16.4% over 2010.”

These holiday season findings are based on data from IBM Benchmark, an analytics system that measures online results from the web sites of more than 500 U.S. retailers. All of the data is aggregated and anonymous.