Thanks for the kind words. Not being a designer of any sort, they are all the more kind.
Having now looked at the iPhone SDK off and on for a while, it seems much more straightforward for Apple to introduce a third development platform. The iPhone differs primarily from the Mac in the touch extensions to Cocoa and a few additions to the Interface Builder's button collection (plus a few other odds and ends). The workflows for Mac and iPhone are surprisingly close. Adding a mid-sized tablet to this mix would be pretty easy. Just keep the touch extensions and a new NIB option in Interface Builder for the particular screen size. Once the developers get the hang of the options made available by multi-touch in iPhone apps, Tablet apps should come much more naturally. Also, if Apple leads the way with a broad collection of their own apps (just like on iPhone), the rest of the development community can come up to speed on the new interface metaphors that much more quickly.
That being said, the main operating system for the tablet could really go either way (custom or Mac). Having thought about this for a long time, I'm leaning ever more towards using the full Mac interface. While the Mac OS has shied away from fullscreen apps, the tablet could embrace this feature (much as the iPhone has) and in this way keep the main look of the Mac while also taking full advantage of using all of an already smaller screen. This paradigm shift alone could distinguish the tablet from the rest of Apple's lineup of portables. It would necessarily complicate this lineup, but at the very least it would create a nice bridge from the iPhone to the MacBooks.
Realistically, Apple is going to want to avoid creating yet another development platform with its own unique collection of tools and interface designs. If I were to venture a guess, Apple is probably none too keen on having the iPhone and Mac platforms as separate as they are right now. They did their best with the dual role Xcode is currently holding down, but I doubt they feel it is enough. If they can pull the touch controls into the main fold, then they can merge the two lines. The tablet I'm hoping for would do just that.
Ultimately, however, there are design challenges when going from 3" to 7" to 15" to 30" screens that no amount of "resolution independence" can overcome. While much of the coding frameworks for a tablet are already in place, the discipline necessary to use the screen space wisely is not. Mac developers are only just getting to grips with the iPhone. It takes time to become comfortable with the new design requirements on such a small screen (especially when the Mac has been moving towards ever larger screens). If Apple is truly optimistic about overcoming these troubles, or overly troubled by the split development path, then they might push forward the tablet this. Until then, we'll just have to wait!
