MYMAC=$(arp -a | awk '/[0-9a-f]:/ { gsub(/^0:|[1-9a-z]0:/, "00:", $4); gsub(/:/, "", $4); print $4; exit; }') && curl --header "Content-Type: text/xml" --data "http://skyhookwireless.com/wps/2005' version='2.6' street-address-lookup='full'>betajs.loki.com$MYMAC-50" https://api.skyhookwireless.com/wps2/location
had to change 0: to 00: and strip out those :'s