Is it possible to run something (e.g. a script) after a certain idle time?

[Chris Green]

I have my system set up so that the Power Manager turns off the
display after about 15 minutes idle.  Is it possible to trigger
running a script in the same way?  I have some actions I'd like taken
at the same time (or similar) to when the display turns off.

I'd guess it might be possible by getting xfce4-power-manager to send
something on dbus which could then be monitored using dbus-monitor.
However if there's a simpler method I'd be pleased to know.

-- 
Chris Green