Is there a Mac OS X equivalent of the Linux iwconfig command?
ifconfig gives information about all interfaces, including WLAN. The WLAN interface is usually en1.

$ ifconfig en1
en1: flags=8863&lt;UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST&gt; mtu 1500
        ether 20:c9:d0:97:22:e9 
        inet6 fe80::22c9:d0ff:fe97:22e9%en1 prefixlen 64 scopeid 0x5 
        inet 192.168.1.137 netmask 0xffffff00 broadcast 192.168.1.255
        media: autoselect
        status: active


airport -I gives information about WLAN interfaces (check out the man page for additional options)

$ /System/Library/PrivateFrameworks/Apple80211.framework/Versions/A/Resources/airport -I en1
     agrCtlRSSI: -68
     agrExtRSSI: 0
    agrCtlNoise: -85
    agrExtNoise: 0
          state: running
        op mode: station 
     lastTxRate: 54
        maxRate: 54
lastAssocStatus: 0
    802.11 auth: open
      link auth: wpa2-psk
          BSSID: 11:22:33:44:55:66
           SSID: serenity
            MCS: -1        channel: 6