Are there separate versions of Windows for 640x480 and 1920x1200?
Nope, and thats the main reason MS has utterly failed to get a foothold in the tablet market so far. The same desktop UI that works on HD monitors with a mouse and keyboard just doesnt work on a touch tablet or a phone.
I agree though that it just should be a different UI running on the same OS. Im pretty sure thats where Google is heading, but they may have rushed things a bit as android was in dire need of a tablet version. In the long run I suspect they will seperate the UI more cleanly and merge the underlying OS again. Either that or they intend to merge honeycomb with chromeos and keep the "other" android for phones.