iCal Events Vanished - Cannot Quit iCal

Yes! That is exactly what the problem was! I went and deleted all my to dos from mail and now its working. I'm happy to have it back...frustrated my to dos are screwed.

awhhhh technology. Guess its back to a pen and paper for my to do list.

I have exactly the same symptoms as the OP after updating to OSX 10.5.6 last night - all my events and calendars are gone, I have the January 2001 mini calendar and the application is unresponsive to quit command or creating a new calendar or anything.
I tried deleting the cache and restarting, that made no difference.
I tried deleting iCal completely, but I can't reinstall because my optional install list won't let me select iCal, in fact it will only let me select iTunes.
I haven't got my backup hard drive with me at the moment - in a fortnight I can see what happens when I restore iCal from that.
For the time being, has anyone got any ideas?

I have fixed my iCal problem! woohoo! I had to use Pacifist to extract iCal.pkg from my OSX install disk, but once I'd reinstalled it opened fine with all of my old calendars and data back where they should be, as if nothing had happened.
Dunno what that 10.5.6 update did to mess it up but it's back to normal now.

Just to add another voice to Apple: This problem is real!!! I have a Powerbook G4 running Leopard, just fine until the last update. Same iCal problem after last update: all events gone, unable to use software, won't even quit.

I've read this thread to this point and have been frustrated with the "Disappearing Calendar" Trick. It has been a real "Apple Products are not without potentially serious issues that remain unresolved" experience, but I've achieved a serviceable, kludgy, workaround.

I had three files that were consolidated calendars from previous calendars.(Work, Union, and Personal) I would import the data from these local .ics files into iCal (3.05, from the latest Combo update of 12/22/2008). I would close the application, re-open and nothing would be there.
I re-imported the data from my local .ics files and then deleted the User Library level iCal files, including caches, then closed the app, hoping to see writeout on quit.
No Joy.
I tried some CalDAV trickery with importing those ics files and Google Calendars, then using Google's CalDAV tool (Calaboration?) to bring those into iCal. Still No Joy, but a smile. Google's effective importation of all of those events suggested that the local .ics files weren't corrupt.
I used Pacifist (1st time using, great tool!) to identify Frameworks and files at different levels of the OS (10.5.6) file system related to iCal and wiped them out. Some of the DAV stuff I overwrote from the install disk. I effectively downgraded my iCal to 3.0.1, did the whole re-importation from the local ics files and still No Joy.

I got iCal to import and SAVE ON QUIT the info from the "Work" Calendar easily. I got it to do the same for my "Personal" Calendar as well. But no matter what I did, I couldn't get it to hold onto and record the info that I imported from my "Union" calendar. I hope that's not a statement about worker's rights to organize and collectively bargain...

Anyways, the workaround that finally brought SOME JOY, was to publish the calendars to ".Mac" and not "MobileMe", than log into my account on www.mac.com (NOT www.me.com!) and download the resulting Calendar file as "Union2.ics". When this file was imported into iCal (downgraded to 3.0.1) the information finally held!

Now on to importing into my iPhone which was the whole point of this exercise..
Moral: .Mac is not the same as MobileMe
Apple has a medium sized bug with iCal
and a little trip over http for your data might not be so bad.

Creating an event on me.com from the me.com website seemed jar something loose. It caused an error stating the next sync would take longer, but after that, I appear to have no more of these vanishing event symptoms.