We definitely looked at the Zune desktop UI as the starting point to do a Metro-like experience in a desktop client. We also draw a lot of inspiration from the Metro guidelines on WP7S, but obviously not all of that is applicable on the desktop.
In areas like the notifications, we're trying some UX concepts of our own with the "Metro" philosophy in mind.
This could be a dumb question but can MetroTwit handle more than one account simultaneously?
Brilliant client BTW. Simple, elegant, stupidly easy to read and use. Well done Long.