Not only will the notification come back, and the accepted event turn grey (tentative), but this also causes iCal to always show the date of the first notification when you launch, instead of today's date. Here's how to kill these zombie notifications permanently:
- Quit iCal.
- Locate and delete the offending notification(s) in ~/Library » Caches » com.apple.iCal » Incoming.
- Open iCal and sigh in relief as the notification is gone for good.

