Detecting in/activity

FROM : Jacob Bandes-Storch
DATE : Sat Nov 15 23:43:21 2008

Hi all,

I'd like to have an app/daemon/something detect when the computer 
becomes active after a period of inactivity, much like iChat does when 
it says "Welcome back! Would you like to change your status from ...". 
Does anyone have any insight as to the best way to do that?

Thanks in advance.