As Trevin stated – “Moments ago we just flipped over Windows Live Mail (aka "Kahuna") over to the new Windows Live ID sign-in UI”.
To get this experience, you must:
- be a Live Mail user, and
- sign in through http://mail.live.com (as opposed to www.hotmail.com). Update those browser bookmarks to the new URL so you consistently get the new sign-in UI.
If you don't already have the Windows Live Sign-in Assistant, what are you waiting for? Go grab the Windows Live Messenger beta from http://ideas.live.com and install it!