Problem: After changing your password on your CalDAV server, you can not reset the password in iCal. After you change the password in System Preferences or in iCal, the iCal app always prompts for you to enter your password every time you access your CalDAV calendar. You password is always incorrect.
Comments: Clearly, this is a caching problem. The old password is being passed to the server. However, attempts to change the password are ignored.
Solution: Clear the cache. The passwords are stored in OS X Keychain. Open up the Keychain Access tool found in the
Applications -> Utilities directory. Delete any references to your CalDAV server. I rebooted for good measure. After that, I connected to my CalDAV server, the new password was cached, and everything worked fine. Woo hoo!