You can use a stylus on the iPad if you want. Dagi Stylus or Pogo Stylus are just two you can buy.
The "Tablets" that have been out for years have never really caught on. There was even a company that made an Apple Tablet (ModBook). They never got popular because they relied more on handwriting recognition (if they were the slate type) and "typing" with a stylus is not at all quick. The convertible types were bulky. And NONE of them were designed from the ground up to use a touch interface.
THAT is why the iPad is different. It's going to bring tablets, finally, to the masses. It's a pleasure to use. It's small and light. It doesn't just feel like a computer you have to touch... it's a device meant to be touched. It brings some of the tactile sensation back to electronic media... something that's been missing.