The design is similar to the one used by other online web clients, like AIM and Apple's new default IM layout in Messages. Instead of the old just-the-messages, click-to-go-back-to-a-list layout, you can now just click on your friends' names on the left side of the screen.
Other additions are being able to embed multiple pictures in a single message, and some new keyboard shortcuts. It's rolling out starting now, so you might not have the new layout right away. But once you do, it might actually be a reason to use Messages on Facebook for once. [Facebook via TNW]