I live downtown and Airport picks up 20-some network signals on a given day. Sometimes it connects to ones without passwords if it doesn't feel like connecting to mine. Is there a way to tell airport to only connect to the networks I specify?
Also, in System Preferences > Network > Airport > Configure > Airport Tab > Options, "If no recent networks are found:", you may want to select " Ask before joining an open network" to prevent Airport from automatically joining open networks.