I, like many other users, have had the mysterious "Mac OS X wants to use keychain system" dialog box after each reboot, when AirPort tried to access the wireless network for the first time, that refused to accept any known password. There have been some users out there who solved the problem by simply removing the System.keychain from /Library/Keychains/. Some other users suggested creating a new System.keychain with a known password.
I didn“t feel comfortable using one of those solutions, because I had the feeling that there must be (1) a reason that there is a System.keychain, and (2) that it doesn't have a keyword that is known by the user. Digging a bit deeper into the system (BTW, I'm a designer and no programmer), I found out that the initial installation of 10.4 (and every subsequent update) contains a postflight script that will create this particular keychain. So my solution was quickly found.
You need to have admin-rights on the Mac you want to update using this hint. Then just:
sudo mv /Library/Keychains/System.keychain /Library/Keychains/System.keychain.baksudo /usr/sbin/systemkeychain -CMac OS X Hints
http://hints.macworld.com/article.php?story=20060318124936642