Please use Modern UI for the moment, because Metro was a code name that conflicts with Metro AG.
Feel free to update questions with the new naming as you go around making our Windows 8 part of the site awesome and geared up towards the first weeks in the Windows 8 era...
Consider to use modern-ui to denote questions specific to applications in the new UI, we can always decide to drop the tag later but for now it helps differ between things like "Skype on the desktop or Skype in the Modern UI?"
Tag request: Synonyms from metro and metro-ui to modern-ui might be handy.
modern-ui
now just creates yet another tag for the same thing.