The latest crop of tablets--including the iPad, the JooJoo, the HP Slate (to name only a few)--springs from over two centuries of research and development, starting with Elisha Gray, whose 1888 "Telautograph" (U.S. patent No. 386,815) is believed to be the forerunner to the modern tablet.
The first tablets, as we would recognize them, didn't come about until the late 1950s and early 1960s. These "tablets" consisted of a large computer terminal attached to a receiver pad, which accepted electrical or magnetic input from a stylus. They were extremely expensive to make and extremely heavy. Over the years, as tablets became functionally more complex, they also became more compact. The rest is history.
Yes, we've come a long way. See just how far by viewing our slideshow of the tablet's evolution since the 1960s.