Detecting ground with Atmel/Arduino and waking up from sleep
Your diagram would work but since battery power saving is a concern, there's probably a more effective way to do this. However my way won't put the atmel to sleep, but rather turning it off and on completely.

You should have a battery and make an open circuit between the negative of the battery and after the load, then put the two wires exposed to ground. And if the ground is made out of conductive metal, when the board is placed on the ground, the two wires will come in contact with the ground and the circuit will flow through the battery, through the load, through the wire to the ground, through ground, back into the other wire and back into the battery.

It should look like this:


You could have some soft button or pressure switch and make the board push the switch down on its own weight, but that is if your ground isn't conductive.