I'm all for a revolutionary new system, but I'm still not convinced that such a leap is necessary or even practical in the near future. When I try to imagine what an Apple tablet will look like, I'm not looking 5-10 years in the future, I'm looking towards this coming spring. With that schedule in mind, Apple would be releasing the tablet alongside the iPhone SDK. Just as Mac developers are starting development on a second platform, Apple would never confuse the issue with a surprise release of a
third platform with yet another SDK! Now certainly Apple would want their tablet to have an optimized OS, but that would develop over time. Being able to run native iPhone apps side-by-side with Mac apps would ease the transition. I see no reason why a small tablet couldn't run regular Mac apps without ANY modifications.
The only way I would imagine Apple could push the tablet as a third platform would be if they built it right into the iPhone SDK. Just as you can turn universal binaries on and off with a checkbox, you could also choose to build for a MacBook touch. All the dev would have to do is draw up a second NIB interface file.
Anything more agressive than that would force developers to target three completely different platforms. Cocoa still hasn't completely killed of Carbon, Leopard apps are just now showing up, and the iPhone apps haven't even been sanctioned yet. It would be silly to complicate the matter further.
--- --- ---
A new mockup below, moving further away from the default Mac look but still in line with iPod touch UI elements: