Once heralded by Microsoft chairman Bill Gates as the future of portable computing, Tablet PCs now appear to be a niche market that has slim prospects for significant growth even through the end of the decade.

Forecasts at the time of the Windows XP Tablet Edition launch expected sales to hit 5 million in 2005, 9 million in 2006, and 14 million by 2007. However, sales so far this year are not even in line to hit one million.

In contrast, a recent projection by IDC said that Tablet PC units should reach 14 million by 2009.

Gartner has also lowered its projections and is expected to release revised forecasts shortly. Gartner projects that between 1.6 and 7.2 million units could be sold in 2009, with 3.5 million units the most likely scenario, similar to Kay's current forecast.

"Alternative form factors have arisen to handle some of the demand that might potentially have gone to tablets, form factors such as Blackberries and converged phones," Kay wrote. "Mobility is clearly important, and yet there are a number of possible ways of getting a good mobile computing experience."

Walt Mossberg, technology columnist for the Wall Street Journal, offered some suggestions on what the Tablet PC industry needs to do to make the units more appealing.

"Until somebody does a better job of designing hardware controls aimed specifically at tablet users, I believe tablets will remain niche products, even when they are sized right," Mossberg said.

Microsoft remains committed to the Tablet PCs and says it continues to invest heavily in the platform, stating that the units are selling just fine.

However, the company would not comment on why analysts have begun to scale back their once-lofty sales projections for Tablet PC systems.