Did you miss that tiny little iCal alert dialog? Use a more prominent Open File alarm.
First save an image, text file, sound, or movie that will really fill your screen and grab your attention.
Select an iCal event and choose Open File as the alert type. Choose your unmissable file as the one to open.
Set more alerts with more files to open for the same event if you like. You won't miss an appointment with one of these alerts!
From Miraz Jordan