I have a similar problem myself. I can unlock the "System.keychain" keychain, but if I want to move items around, it asks me for the password again.
When I press the unlock button, it asks for the password for my full user name. And that one works, the keychain unlocks. But an unlocked keychain doesn't really let you do much administration on it. You must enter its password for any administration tasks. And that's when I get stuck: the system doesn't accept any password I know of, not even the empty password.
But I can delete items. So I can delete the item, then recreate it in the other keychain.
It seems like the system keychain is like a group, that can contain references to keys so that the whole system can use that key. But is it really like this, or is a copy of the key created, when a key automatically gets into the sytem keychain (like the airport keys)?
PowerBook 12" Mac OS X (10.4.4)