In case you wish for certain events to take place on sleep or wake of you Mac, the software sleepwatcher is your friend. You can install it from Macports with the following line:
sudo port install sleepwatcher
To launch it on system startup, do this:
sudo port load sleepwatcher
If e.g. you want to show a login window on wakeup, create an executable file called ~/.sleep with the following contents:
/System/Library/CoreServices/Menu\ Extras/User.menu/Contents/Resources/CGSession -suspend