For the past week or so, I have been testing a sleek, light, silver-and-black tablet computer called an iPad. After spending hours and hours with it, I believe this beautiful new touch-screen device from Apple has the potential to change portable computing profoundly, and to challenge the primacy of the laptop. It could even help, eventually, to propel the finger-driven, multitouch user interface ahead of the mouse-driven interface that has prevailed for decades.

But first, it will have to prove that it really can replace...