If you want to try the new Nautilus, Ubuntu Software Center and Software Updater icons (not released yet), which will be available in Ubuntu 13.04, you can use them right now on any Ubuntu version thanks to a new icon theme available on gnome-look.org called 'My-humanity'.
My-humanity icon theme uses the new Nautilus icon for the folder icons as you can see in the screenshot below:
These aren't the official folder icons that will be available in Ubuntu 13.04 - they are custom icons built on top of the Nautilus icons we've seen a few days ago.
Besides the new folder Ubuntu Software Center and Software Updater icons, My-humanity comes with some other custom icons, so it's not identical to the Humanity icon theme available by default in Ubuntu. And it's pretty interesting!
To use My-humanity, extract the downloaded archive, copy the "my-humanity" folder in the ~/.icons folder (".icons" is a hidden folder in your home directory - if it doesn't exist, create it) and then use a tool like GNOME Tweak Tool, Ubuntu Tweak or Unsettings to change the icon theme to My-humanity.