Google started rolling out their updated user interface for Gmail a few days ago. The changes are subtle, there is no eye-candy (à la Vista or Leopard), but it's all great stuff.
Here is my favorite: when you open an email, Gmail adds a fragment identifier to the URL. Now you can bookmark this URL, or copy it somewhere and reopen it later to get exactly to that message.
You could get to the same result before by opening the email in a new window, but it was much slower and not as straightforward. I love it!