I begin with the disclaimer that this will only work with young children or those who won't question how things work the way they do, or rule followers.
In the interest of encouraging Ethan to be a little more independent and allowing him more access to the computer, I recently set up parental controls on my computer (the ones that came with Windows Vista.) In doing so I discovered that I could set the hours during which Ethan's user account is allowed to use the computer. No more fighting with him to get off of Webkinz World and come eat dinner at 7. Now the computer shuts him off at 7 and he comes downstairs on his own.... WOW what a change it made in my evenings!!!
If only the Wii had a parental control like that -- I checked, it doesn't.
Then today the clouds parted, angels sang, and I had an inspiration. I went to Wall-mart and bought one of those auto-timers you use to automatically turn on and off your lights or TV in your house when you are on vacations... ($4) I plugged it into the power strip, set it to turn on at 7 AM and shut off at 7 PM then I plugged the Wii into it. Now it will automatically shut off the power going to the Wii machine at 7 PM. I set the clock on the DVD player so Ethan can plan ahead and save his game prior to shut off.
If we want to play a family game after 7 we can just plug the Wii into one of the other slots on the power strip.
It probably wouldn't work with older kids, but it will work for my child - he challenges a lot of things... but if you tell him the machine is programmed to shut of by itself, and then it does... He doesn't question technology. I can't believe the solution was so simple and it took me so long to figure it out!