I just ran across this nice set of Longhorn UI related articles, officially titled Aero User Experience Guidelines: Sampler for PDC 2003.  Links to the meat are under What's Inside? heading.  Some good ideas, some questionable, but all good looking.  I am starting to get a little sick of all the gradients though and the huge titlebar seem wasteful.  I wonder what usability tests caused them to make the title bar much bigger?