Interesting discussion here, thanks. Sorry if my previous comments felt dismissive, I would hope some other words would characterize them better... "realistic", for instance.

First of all, there is nothing that cannot be done, given enough effort. However, I didn't try to talk just about the basic differences... Yes, you can do a system with a reasonable effort that shows different size icons and buttons for the UI for stylus and finger, but at least I'm not a fan of doing things halfway, I tried to talk about "optimal" UI's. Optimal UI's should not feel like compromises.

Information density is basically split in half for finger vs. stylus, meaning how many controls or UI elements you can put on the screen. If you have a plan how you could design a system-wide style that would automatically accommodate the UI's so that the amount of information and the amount, size and placement of the UI controls in any given view would work for both stylus and finger optimized formats, please let me know about it.

My full quote tried to be: "It is not possible as a system-wide issue to design UI's for all the applications that would be 'optimal' for both stylus and the finger at the same time, unless nearly doubling the design and implementation effort." (emphasis added).

Yes you can do it by doing the design and implementation twice, and yes there are some small parts which you don't need to design twice, hence the "nearly". But just trying to squeeze stuff, have the same design, same components and same thinking for each of the UI views, will not be optimal for both stylus and the finger. The stylus does permit to place "more stuff on screen", whereas for the finger minimizing UI clutter and therefore having a bit more hierarchy works better. (Of course it's not as simple as that, but...)

A finger optimal UI won't be stylus optimal UI and vice versa. At least platform-wise, since people are going through different applications, in and out, switching between them, doing different tasks, different views and windows, having some apps stylus-driven (hey, i need to take the stylus out) and others finger-driven (damn, i need to put it back in) will not create a satisfying user experience.

Also the example about advancing touch UI's further with things like multitouch (nothing announced, sorry ) run contrary to trying to hold on to a dual stylus/finger UI style. (And multitouch is just one example, not a main driver.)

But also as noted in some of the comments here, nothing basically prevents "you" as the developers from doing otherwise.