Same here, except the error message is popping up every 10-15 seconds as I work in iCal. If I stop making changes in iCal, it stops popping up.
Needless to say, this is REALLY annoying...
Notable events after which it began:
1) publishing some calendars to the web
2) adding an iPod to my machine (just got it)
3) opening and closing iSync
Specs:
G5 dual 2.0 PPC
OS X 10.4.8
iCal 2.0.4
Any thoughts?