Mac OS X ships with a command-line tool called airport that can do all sorts of nifty things with Wi-Fi networks. Unfortunately, it's so squirreled away that most people don't seem to know about it. The utility is part of the Apple80211 Private Framework used to power your Mac's Airport menubar icon.
Invoking the utility without arguments prints a useful (if incomplete) usage message. At a Terminal command prompt, type:
/System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport
sudo ln -s /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport /usr/bin/airport
$ airport en1 scan SSID BSSID RSSI CHANNEL HT CC SECURITY (auth/unicast/group) moscohome 00:22:6b:8b:86:51 -61 10 N -- WPA2(PSK/AES/AES) PUBLIC-455H 00:15:6d:60:95:d1 -82 1 N -- NONE Alex Network 00:1e:e5:24:c4:4f -86 1 Y TW WPA(PSK/TKIP,AES/TKIP) WPA2(PSK/TKIP,AES/TKIP) linksysELNIDO 00:21:29:a3:fd:99 -90 6 N -- WPA(PSK/AES,TKIP/TKIP) WPA2(PSK/AES,TKIP/TKIP) 2WIRE024 00:18:3f:02:2f:49 -88 6 N US WEP 2WIRE940 00:12:88:d9:85:41 -93 6 N US WEP
airport en1 scan | grep WEP
More awesome, perhaps, is the tool's ability to actually perform traffic sniffing and capture packets. Tell airport to sniff, and optionally provide a channel (which you now know thanks to your ability to scan). You need to be an administrator (i.e., you need sudo privileges) to do this:
sudo airport en1 sniff 6
This creates a file called airportSniffXXXXXX.cap in the /tmp directory, where XXXXXX is a string for uniqueness. You can then feed this file into your favorite network analyzer such as Wireshark to examine the traffic offline.
[crarko adds: I tested this, and it works as described. Things like this and Wireshark are powerful tools in the hands of a network administrator. I trust I don't need to remind you to use your powers for good, and not for evil.]

