Worldwide sales of tablet computers are on pace to reach 63.6 million units according to a recent report from Gartner. If correct, tablet sales would be up over 250 percent from 2010 sales of 17.6 million units.

While the increase is still amazing, the numbers are down significantly from the almost 70 million in predicted sales in Gartner's April report. The firm has significantly lowered its sales forecasts for Android, WebOS and QNX tablets.

Gartner now believes the Apple iPad will account for 3 out of 4 (73.4%) of worldwide media tablet sales in 2011, down from 83 percent share in 2010. Gartner has lowered its sales forecast of iPad tablets by about two and half percent since April.

Beyond Apple iOS and the Android operating system (OS), Gartner does not expect any other platforms to have more than 5 percent share of the tablet market in 2011.

“We expect Apple to maintain a market share lead throughout our forecast period." said Gartner VP Carolina Milanesi.

The firm believes about 11 million Android tablets will be sold this year. That would give Android tablets 17.3% of the market up only slightly from Android’s 2010 market share of 14.3 percent. Gartner’s forecast for the Android OS has been lowered by 28 percent from last quarter’s projection. The reduction would have been greater had it not been for the success of lower-end tablets in Asia, and the expectations around the launch of Amazon's tablet.

“So far, Android’s appeal in the tablet market has been constrained by high prices, weak user interface and limited tablet applications” Ms. Milanesi said.

Gartner analysts said Research In Motion’s QNX OS is a promising platform, but it is still in the early stages of development. RIM’s main challenge will be to attract more support from application developers as the company is going through a tough period, with considerable pressure on its smartphone business.

Gartner has reduced its 2011 sales forecast for QNX tablets by almost 25% from 3.9 million in April to 3.0 million today. It's forecast for the defunct WebOS tablet fell from 2.8 million in April to 2.0 million today.

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