Just finished cutting and assembling this today. The junction box contains a dimmer, an outlet, and a servo. Top socket supplies power to the arduino while bottom socket provides power to a lamp of some sort (in this case a work lamp, mostly for demonstration purposes.)
I don't want to give away the ending, but once I finish the assembly of the remaining 2 parts, the entire arduino rig will be contained inside the junction box, making for a really nice, clean, finished product.
(Full blog post here)