Ok, so basically you have no really tight power constraints. The only issue you should take care of is to limit the power consumption of you regulator.
Besides that you might want to follow this example:http://interface.khm.de/index.php/lab/experiments/sleep_watchdog_battery/
With regard to cutting the power of the ethernet shield: this can be achieved with a suitable transistor.
Of course you will need to tweak your hardware a little bit but this is the price for buying the hardware before researching the problem. It's considered part of the fun