Where Are Wireless Passwords Stored on Mac OS X?

By default, Keychain Access shows the login keychain. If the sidebar does not show any other keychains to choose from, then hit Command-K (menu View » Show Keychains) to see more choices.

On my Mac, both the login keychain and the system keychain show the same AirPort Network password, though the latter is a few years older. The first seems to be used by name and the latter by MAC-address. When looking at the Access Control to see which application can access the passphrase, then one sees some differences as well. But for another Wifi network I only have an entry in the login keychain. (Maybe the system keychain was used in previous versions of Mac OS X, but is still used if no entry in login is found?) I guess deleting all that matches indeed seems the best thing to do.

(As a side note: when one has mounted some network drive with limited credentials, and one wants to change that credentials even when the old ones are still valid, then one also needs to search in the system keychain, not in the login keychain.)

Check in Keychain. All the systems passwords should be there. I know it's not what you want to hear, but I doubt the passwords are stored anywhere else; that's the purpose of the Keychain. I haven't heard of any replacement for it being used in Snow Leopard.