No external device needed, just root access and I usually make a symlink to somewhere on $PATH:

  $ sudo ln -s /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport  ~/bin/airport
  $ sudo airport sniff
  $ tcpdump -r /tmp/airportSniffxxxx.cap | less

It sure does, which is awesome! The UI even has support, and when it’s in monitor mode the WiFi status icon even turns into a solid fan shape with a little eye in it. It’s the little things.

‘jzelinskie has the short instructions in their comment.