The HP Folio 13 Ultrabook
Just got mine in the mail today (it's a gift for the girlfriend). HOLY CRAP this is a gorgeous laptop. Very well executed, materials are great, keyboard is great, trackpad is great. Don't know much about performance yet.
Definitely thin, although maybe not as much as other ultrabooks.
So far, of the Windows Ultrabooks I've played with, I like this one best. The Toshiba, while light and thin, is so flimsy and cheap feeling I'd never own one. Asus is nice, and on par with the Folio. I own a Macbook Air 13, and I say the Folio's build quality is right up there with it.
Anyway, just thought I'd share. This laptop was a steal for the ~$700 I paid for it from HP!