I don't know who is actually supposed to buy Tablet PCs, but I'm guessing it's not engineers or graphic artists, at least not as their main computer. The nicest thing about the keyboard and the mouse is that you don't have to rely solely on either one, when it isn't well suited to the application.
On to the idea...
The important part is a bar, connected via USB, 25-30cm in length, 8-10cm wide. There is a three axis knob (x,y,z or x,y and rotation) at either end, ringed with eight or so buttons. In the middle is a numeric pad with a few additional, stylized buttons. Ctrl, shift, alt, backspace/delete, numlock, that sort of thing, shaped so as to make the keypad round overall.
The second part is a one-size-fits-all carrying case for a tablet, with a fold over screen protector/stand. Rubber balls at the corners, since it's intended for field work; engineers working on-site, or graphic artists on a rare jaunt throught the outside world.
You strap the tablet into the case, and then dock the input bar thingy to any of the four edges. Rotating the keypad into the correct position sets the orientation of the three axis whatsits.
Thus, full 3-d manipulation of objects in CAD or 3D graphics; color/saturation/etc control and a quick access layers/menus control in 2D, and a directional pad (what is this called? the arrow buttons) via the numlock for fine cursor movement.
The input bar can be removed for desktop use. The device might appeal to some gamers as well, possibly hooked into a regular laptop or desktop.
Although, since christmas is so near, I might as well dream about a custom Tablet integrating this device and a capable graphics engine.