This plugin is available in the 'plugins' folder in src dir of latest svn. (If you can't see it, make sure you have 'svn up' -ed to the latest revision). Located here.

Installation instructions are provided, basically copy everything in the /plugins/Rhythmbox/ directory to /home/YOUR_USERNAME/.gnome2/rhythmbox/plugins/ . Reboot your computer. Then restartstart Rhythmbox, load the plugins chooser (Edit->Plugins), and select the 'artdisplay-awn' plugin, then watch as Awn amazes you by showing album art ;).