A while back, it was decided that Ubuntu 13.04 Raring Ringtail will ship with GNOME 3.6 for the most part instead of GNOME 3.8.
Raring Ringtail might become Rolling Release but if/until that happens, Raring users can add the GNOME 3 PPA, which was updated recently, to upgrade to the latest GNOME 3.8 beta for many components, including GNOME Shell and Nautilus.
Additionally, a second PPA (GNOME 3 Staging PPA) has been created recently and allows Ubuntu 13.04 Raring Ringtail users to upgrade other components that aren't available in the main GNOME 3 PPA. The GNOME 3 Staging PPA packages are work in progress or have known regressions so it's not as stable as the main GNOME 3 PPA. Also, if you want to use the GNOME 3 Staging PPA, you'll have to firstly add the main GNOME 3 PPA.