Apple has released a "Lights-Out Management Firmware Update 1.1". This allows the machine to be remotely turned off and on via the ethernet ports.
This update can be performed without restarting the machine, even remotely. But be careful, as it would be better to be nearby in case there is a problem and the system crashes, which would require a reboot of the machine.
To acquaint yourself with the more or less complex procedure:
http://docs.info.apple.com/article.html?artnum=305367
[translation by jwa]
