A Good question.
I don't think currently there is any way to shut down completely without removing the battery.
It will try to wake up when next alarm from the clock or calendar is due.

This feature could be usefull though; or setting "ignore alarms when in shutdown"

Also a feature which would make the device automatically boot up when certain type of alarm is trickered. N900 could be used remotely to boot up once a day/week in some remote location for few minutes to see if there is commands to be executed or not, and then shut down for the next alarm.