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First... It should be compatible with different hardware platforms.
Second... it should be easily expanded
Third... it should be easily maintained/updated.
I could not agree with you more, strandtc. In fact, I had tried steering some people in this direction a couple of years ago. They of course opted for the tightly integrated app, and today it is another piece of vaporware. Give me a few hours with a linux desktop and I can customize it to make it as touchscreen freindly as any of these simply by choosing the right combination of window-manager, toolbars, docking applications, etc...
Look at OPIE and GPE for handhelds as an example. They look nothing like KDE and GTK, yet they are able to run many of the same apps with only a few modifications to make them suitable for PDA use.
Going this route does not mean that your system has to look like a desktop either. Using X11 and a window manager does not mean that you have to have a "start menu" in the bottom left hand corner. If you want your application menu to be a full screen app that looks exactly like the one for nghost, you can do this.
OpenMoko is a good example of something that runs on X11, yet looks nothing like a desktop.
http://wiki.openmoko.org/wiki/Main_Page