A video from Google’s YouTube channel (update: the video is now private, but there’s a mirror here) introduces a new Gmail interface. It’s based on the Preview theme that’s already available in Gmail, but there are many other changes: an action bar that uses icons instead of text labels, a completely new interface for conversations, profile pictures next to contacts, a flexible layout that adapts to any window size, display density options like in Google Docs, resizable chat/labels sections, new high-definition themes and an updated search box that includes advanced options.
I don’t see the new interface yet, but it will probably be available soon. It’s interesting that there’s an arrow next to “Mail” and the links to Google Contacts and Google Tasks are missing. Maybe you’ll be able to use the arrow to switch to other Google services.