I've always thought that a tablet Mac would only have limited appeal - great for real estate agents, medical professionals, and some others, but impractical for everyday use by most people. But Jeff Han's demo and the iPhone have me rethinking that.
The main issue, of course, is input: a finger or stylus is fine as a pointing device, much like a trackpad or mouse... but text input isn't as practical. Sure, Apple has Inkwell, which supports handwriting recognition, but typing on a keyboard will probably always be faster for most people.
But while there is definitely some advantage to the tactility of a hardware keyboard, that may be mitigated by the versatility of a software keyboard - displayed on-screen.
I think that this may well be the direction Apple will head.
I imagine a future iMac as a 30" panel angled at about 45° from horizontal (adjustable), with the computer guts hidden underneath. There is no keyboard, no mouse - just a large screen right in front of you, like an architects drafting table. You interact with it with just your hands - no stylus or other hardware.
Like in the iPhone, you can scroll with the flick of a finger, "click" or double-click just by touching, and use multi-touch gestures to zoom, move, resize, and even rotate the screen content.
As in the picture manipulation in Han's multi-touch demo, windows in the Mac OS X of the future would float in three dimensions. You can zoom windows forward or back, drag them around (perhaps via touches of their titlebar or empty space, like modern textured windows), etc. The windows scale smoothly via resolution independence. There could be a button somewhere on the screen (or a hardware "home" button like on the iPhone) to show all of the windows, like exposé, allowing quickly finding a specific one.
Controls within windows would work the same way: flick scrolling, pinch zooming, finger dragging, and more.
As I mentioned, the biggest issue for me has always been the keyboard... but I'm coming around to the view that a software keyboard could be an entirely feasible replacement. The keyboard could zoom into view when you need it, vanish when you don't, and be reconfigured to suit the application. A numeric keypad with special function keys in calculator and spreadsheet apps, a full qwerty keyboard in a word processor, and other variations. They keyboard could even be scaled and moved around as needed. To compensate for the lack of tactile feedback, it could play tapping sounds when keys are pressed, or even speak the keys or words typed.
It'd probably still need to have menus at the top of the screen, but maybe some sort of contextual replacement could be devised. Similarly, it might still have the Dock, but it'd be zoomable and much more flexible.
Reminds me of Apple's Knowledge Navigator concept video... from 20 years ago.
We could even carry it further, perhaps for more portable Macs: perhaps something like a small unit that projects the keyboard and the user interface, using spacial sensing to detect your fingers. This would allow a pocket-sized device to have not only a virtual keyboard, but a virtual screen as well, perhaps several times larger than the device itself.
Of course, none of this is new... and the technology all exists today. If anyone can put it all together in a way that works, Apple can.
I can't wait.