Everything working fine here, too. iCal with lots of entries running just fine. Address book with 92 entries also running fine. Mail, Safari, iTunes all okee-dokey as well.
I stumbled across this problem this last hour and if it hadn't been for you I would still not know what to do. I went as far as to delete the .plists, logging out and in again, trashing the user/library/calendars file and even restarting, with no luck.
For those having problems with iCal post 10.5.2 upgrade, delete the cache files from [User]/Library/Calendars. iCal will then automatically upgrade your calendars.
As embarrassing as it may be to ask, I figure it'd be best to not do something wrong... I see deleting the cache files seems to solve the problem, but what specifically does that mean? I open Calendar Cache under Calendars and get a terminal window... I don't usually poke around with this but I guess I need to start somewhere..
For those having problems with iCal post 10.5.2 upgrade, delete the cache files from [User]/Library/Calendars. iCal will then automatically upgrade your calendars.