The idea for the iPad came from an engineer at Microsoft, who was married to a friend of the Jobs family, and who invited Jobs to his fiftieth-birthday party. As Jobs tells Isaacson:
“This guy badgered me about how Microsoft was going to completely change the world with this tablet PC software and eliminate all notebook computers, and Apple ought to license his Microsoft software. But he was doing the device all wrong. It had a stylus. As soon as you have a stylus, you’re dead. This dinner was like the tenth time he talked to me about it, and I was so sick of it that I came home and said, “Fuck this, let’s show him what a tablet can really be.”
via Steve Jobs’s Real Genius : The New Yorker.