Hold down the Option key when you click on the AirPort icon in the menu bar:
You can then see the SSID, channel, security, RSSI, transmit rate, etc. The airport command is now located at /System/Library/PrivateFrameworks/Apple80211.framework/Versions/A/Resources/airport so this would be similar to:
/System/Library/PrivateFrameworks/Apple80211.framework/Versions/A/Resources/airport -I
