If you've upgraded to 10.4.2 and you find you can no longer use the built-in "Quick Search" field (at the bottom of the screen) in iCal to find your events, to dos, etc., here's the solution: Delete iCal's Index files -- it seems they have become corrupt.
First, quit iCal if it is running. Then, from the Terminal, run the following commands:
$ cd ~/Library/Application\ Support/iCal/Sources
$ rm */Index
If you are uncomfortable with the Terminal, you can also do this via the GUI. From your home folder, open the Library folder, then open Application Support, then iCal, then Sources. Now open each folder within Sources and delete the file named Index (or you can do a search for the filename Index from within the Sources folder -- but make sure you limit it to only the Index files that are within the Sources folder!.
Now launch iCal, and it will automatically rebuild the Index for each calendar. All credit for this solution goes to Bob Burger, who not only first posted this problem on Apple's Support Discussion web site, but also solved it!
[robg adds: This is not a problem for everyone -- my iCal was working fine in 10.4.2. Nonetheless, I tested this hint, and it works as described, rebuilding the index files when you launch iChat again.]

