The Airbook convinced me to give up my desktop and move over to Mac OS. Initially it was the best hardware (given my requirements for light weight) to run Windows 7 (VMWare something or the other).
Now, I rarely load up Windows, unless it is to test something or run a Windows only util.
The Newton (and Psion Organiser) did but not the Palm Pilot. The first smartphone was the Simon Personal Communicator introduced in 1994.
Even you don't consider that a smartphone the Nokia Communicator was released in 1996, one year before the Palm Pilot.
Tablet PCs (a term coined by MS) existed 10 years before the first iPad came out, running OS such as 'Windows XP for Tablet PC'. They were mainly for business use, expensive, heavy and required the use of the stylus but they are tablets nevertheless; the "Post-PC" tablets are a bit touchier and lighter etc., and more succesful obviously.