1.) You could try going into keychain Access. Look for the network name (it's kind will be "Airport network password") delete that and the network should prompt you for a password again.
2.) I've had troubles getting machines running Leopard to connect to a WEP network without the hex password. Unfortunately I'm not familiar with the AT & T router in question so I won't be much use there... can you switch it over to WPA? Leopard connects very well with that & it's far more secure.