Considering the shield is directly attached to the Arduino, I assume powering it manually is impossible without removing it and manually wiring the shield to the Arduino board. This is not ideal as it adds to the bulk/mess of the unit (part of the reason we opted for the shield to begin with). Although we have not ruled out this approach, it is not preferred.
So it would appear that there are not any obscure power saving features built into the WiFi shield itself (akin to those built into the arduino).
I did have the thought that if I just disconnected the wifi from its access point, right before the Arduino was put to sleep, it might reduce power. I would assume that while the shield is unconnected, it is sitting idle and the radio/device would consume far less power. But would it be enough? Has anyone tested the power consumption of the WiFi shield while in various states?