If you're a Growl fanatic, or maybe iCal's alarms just aren't cutting it for you, a forum thread
at cocoaforge might provide you with some options for getting iCal and Growl to shake hands. Discussion has included various methods of using AppleScript, hacking the innards of iCal and even replacing the GUI option of emailing a reminder with sending it to Growl. Either way, it doesn't sound like any of the methods are for the meek of heart, so if you don't feel like digging around in application bundles and AppleScript, you might want to wait for a prettier solution.
[via Hawk Wings