Recently i had to change my WLAN key. Since then my Apple laptop refused to automatically joined the network, although the key was saved in the keychain.
Today i found the cause. If a network with the same SSID but a different key is already saved in the System keychain, Apple stores the key in the users keychain without prompting to replace the existing key in the system chain. And it looks like the system keychain has higher priority so it always tried to join with the old key. Argh.