This may or may not help you, but a similar thing happened to me.
The reason was that, unlike the iSync installer, the iCal installer does not install iCal on top of the older version unless it is in your root applications folder. This means that if you have iCal stored anywhere else on your system, your in for trouble.
I stored iCal in my home applications folder, so when I updated I had two versions of iCal, and it would quit on launch.
All I had to do, was manually replace the older version with the newer one.