The next big thing that wasn't---or was it? IS IT time to declare the demise of the handheld computer, also known as the personal digital assistant (PDA)? A lot of people suddenly think so, for despite high hopes that the devices---made by such firms as Palm, Sony, HP and Dell---would someday become ubiquitous, annual sales have stayed flat at around 11m units worldwide. This compares poorly with PCs, around 130m of which are sold every year, and mobile phones, with sales of around 460m units. ---The PDA market will never be a mass market,--- says Cindy Wolf, an analyst at...