A thread on Cocoa|forge discusses some ways 
in which you can hack iCal so that it will display its alarms through Growl.
Suggestions centre on modifying iCal’s internal applescripts to send iCal notifications to Growls Helper.app so that Growl catches them, following a modification of the instructions in an old macOSXHints, which routes notifications through sendmail.
A further interface hack replaces the “Email” alarm option with a “Growl” one.
The downside to this is that you sacrifice the ability to send notifications via email.
It seems like a lot of work to me, but if you are into that sort of thing – and never forget to back things up first – you will enjoy the challenge.
