One thing that bugged me about iCal was that until you launched it, the iCal icon in the dock appeared as if it was always July 17 ... so if you had it stored in your dock, it was occasionally confusing to see the wrong date on the icon (once launched, the iCal icon shows the current date). To fix this, I thought I'd just edit the number out of the icon and paste it back over the original, but Apple was nice enough to provide exactly what I was looking for without any extra work.
Show the contents of iCal (control-click and Show Package Contents), then navigate to Contents -> Resources, select iCal-Empty.icns and then do a Get Info (command-I). Copy the icon, go back and Get Info on the iCal application itself, and paste the new blank icon over the default. It may look bad for now (the alpha channel wasn't working for me), but all should be normal if you log out and in again (or maybe just relaunch the Finder).
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020914090559339