
Sorry, but your post reveals a lack of knowledge of the PC market when it comes to tablets. Compaq, HP, Archos, Toshiba, Gateway, Acer, etc. have all been making tablets over the last 8 years or so. While none of these tablets have replaced the traditional desktop/laptop format, that doesn't make them failure. Microsoft has made money on the OS, and the OEMs have made money on the hardware.
The tablets themselves are also pretty successful at what they do. I have an HP TC 1100. Its pressure-sensitive interface allows me to draw straight onto the screen, in photoshop, exactly as I would with a pencil & paper (so I can shade, press harder to go darker, softer for lighter, etc.). But I still have two desktops (one Win7RC and one Ubuntu Linux).
The problem with Apple's rumoured tablet is where to pitch it. If it only runs the iPhone OS, then what's the point? You can't run Photoshop, CorelDraw or Painter (or any other truly useful software) on the underpowered iPhone OS, so one of the biggest reasons to buy one (the ability to draw on the screen) as just evaporated. Sure, you can still play with it like you would an iPhone, but then why not just buy an iPhone?
On the other hand, if it runs a full copy of Snow Leopard and has a pressure sensitive touchscreen, then it will cost around the same to manufacture as a macbook (assuming all the other specs are inline/lower than the macbook). Which will make it significantly more expensive than current Windows tablets that have higher specs and can run all of the same or better software, and cost less.
Which begs the question, why would anyone buy an Apple tablet? The answer, as ever, is the fanboi mentality.