Last night I finished Prototype 2 of my Arduino irrigation remote control rig. Added the screw terminal headers so I can connect the irrigation control wires. The very last screw terminal on the right with the white wire is my power bus. It's wired into the COM terminals of all the relay boards.
For those of you who may be wondering, this iteration is mounted on an old IKEA cutting board. I needed the extra physical strength, because the 18 gauge wire used for the irrigation wires is stiff and heavy.
I drilled holes in the cutting board that are just big enough for 4-40 machine screws to poke through (7/32 drill bit, I think). The relay boards have long pins on the bottom, so they need be elevated off the board. I found some 1-inch spacer nuts at Fry's, and that's what the relay boards are mounted on top of. It worked so well, I will be doing the same for the Arduino ADK and Ethernet Shield.
Next step: Wire it up to my irrigation control box and try real hard not the burn the house down. The real trick will be to tap into my irrigation controller's 24-volt power supply without disabling the controller. Fun times!