Green-IT: How do you deal with poweroff systems in your system monitoring?

The easy way:

There are built-in filters for the status view, at the top of the page. You can just have the admins watch "unacknowledged" problems, or problems on hosts that are not in scheduled downtime. Or any other number of combinations.

If you really want to go wild with filtering the CGI view, see the "HOST AND SERVICE FILTER PROPERTIES" section of cgiutils.h in the source code for a full list of filters that are available.

The hard way:

See the docs on adaptive monitoring. With this, you can change the nagios conf, on the fly, as systems are automatically powered off/on. For example, you can adjust the check periods, change the check commands to a check_dummy variant, enable/disable event handlers, etc.