I would like Nagios to execute a Bash command/script when it detects a host down or up. This would allow me to react to down hosts to some degree which would be very interesting.
How would I do this ?
Event handlers are your friends: http://nagios.sourceforge.net/docs/3_0/eventhandlers.html.
into your commands file add the command you need
into your service defination
the arguments that you receive into your script are the ones defined into the command file, you can see them as well by doing echo $@ into your script - is important to know the host address in case that you will call against a remote host and the service state (you take different actions on CRITICAL, OK or WARNING)
i suggest you have some logging into your script at least at the beginning.