I've never had a problem with this. With iChat open and running in the background, anytime my computer connects to a wireless network it will automatically sign online with iChat.
Right on. If you open it when theres no conenction, it will stay saying 'connecting...' as the status for a couple minutes, then it will go to 'disconnected'. If a connection presents itself, and you have already put in your password, it will connect automatically.