% sudo tcpdump -Atq -s 0 -i en1
-i en1will display traffic on your airport card. Use
en0(or nothing, for most systems) for built-in ethernettage.
You can add a host line to limit output to a particular host% sudo tcpdump -Atq -s 0 -i en1 host zombo.com(thanks to Dan Jalkut for this one)