Gmail Gets First Major Facelift With New Themes
It's just a small part of a Google-wide effort to deliver an experience that's "more focused, elastic, and effortless across all of their products." The sneak peek includes Preview and Preview (Dense), which showcases the new interface. Google says the interface will eventually expand dynamically to accommodate different screen sizes and user preferences, but until then you can pick the information density that you prefer.
Google says it's cleaner and more modern, but a lot of avid users are griping that the spaces between emails is now too large. Google admits that a few Labs features act odd in the new themes, but that they'll be working to fix them. So, try 'em out, and see how you prefer (or don't prefer) them. Hopefully Google will allow users to keep the "old" look if they so choose.