This happened to me as well when I upgraded straight from Tiger to Snow Leopard. Everything transferred fine except iCal. (Come on Apple! You're supposed to make **** like this seamless!)
I tried everything suggested in the following articles- none of them worked.
http://support.apple.com/kb/TS1906
http://discussions.apple.com/thread.jspa?threadID=1364074
http://support.apple.com/kb/TS1880
I do have Backup, but I couldn't even restore my iCal from that after trying multiple times from different backup files in multiple ways (so don't feel bad you didn't have Backup- it's not working anyway!).
After searching endlessly and trying everything, the only way I was able to get my Calendar back was to get it from my iPhone (which I'd been avoiding syncing because I was afraid my Mac might overwrite its calendar with emptiness). It prompted me by saying there was a big discrepancy and what should it do (I told it to proceed with adding events to my Mac's iCal). So that worked when nothing else would.
But maybe some of the above articles will work for you. Just wanted to get my experience out there in case anyone else is in the same situation.