Bear in mind that most people think of Windows XP being any good is because their other points of reference are Windows 95/NT (or God forbid, Windows 3.1), which is a bit like saying herpes is good because it is a vast improvement over having syphilis.
I use Windows XP on my work laptop, which when docked is useable(-ish) at 1280 x 1024 resolution on a 21" screen. Undocked, in widescreen 1280 x 800, is just a lesson in frustration (and it's not just the number of pixels, but the physical size of the screen).
If you think that the UI experience of Windows XP will scale pleasantly to a 9" screen (or less), then you are smoking crack.
... and if you are'nt going for the Windows XP UI experience, why on earth should the operating system underneath matter at all?
... unless you are a manufacturer trying to integrate and optimise said operating system onto your hardware for maximum battery life and performance, at which point it really is much more convenient to have the source code*.
The above is presumably why the Intel MID platforms (running on Atom) have so far had wierd and wonderful UIs, with linux underneath.
* Bear in mind these will be price conscious devices: having access to (and the ability to change) the source code of the software the system you are designing means you can optimise it to run on your hardware... and then you can optimise that for performance, power consumption and cost. This is the opposite of what happens on the desktop- where hardware is designed in order to have the resources to run a "one size fits all" OS that is largely ignorant of the specific hardware features it is running on.
Dead Vulture: because that's what Windows XP experience will be like if it ever gets compared to a UI and OS actually designed for these form factors...