Ubuntu 13.04 (Raring Ringtail) is set to launch in April 2013, but the development team has chosen to reveal the official default wallpaper.
It's not clear why the default wallpaper has been revealed so early in the development cycle. Usually, this kind of decision goes through a process in which the winner is chosen. This version may not be the final one, but it sure looks like some kind of evolution.
Canonical has been very conservative with its choice of wallpapers and tried to remain consistent with the evolution of the background, never taking too big steps from one version to another.
The new wallpaper has been announced on the Launchpad by The Ayatana Project. If you are wondering who they are, then you must know that Ayatana is the collective project that houses user interface, design and interaction projects started by Canonical.
They are responsible for designing some important aspects of Ubuntu, such as the Application Indicators, the Me Menu, the Messaging Menu, Unity, and many others.
The wallpaper can be downloaded from the official announcement.