I don't know how deep you wanna go, but the current communication protocol between the shield and the Arduino library does not provide the functionality to access any packets other than TCP (UDP isn't enabled too). So, yes, you'll have to code on the AT32UC3 level but keep in mind, this is not an Arduino platform, it's a 32bit AVR processor with a completely different tool chain.
I would like to be able capture Probe Request or Probe Response kind of packets. I looked at code on git however I still don't understand if I modify this code, will enable me to capture above mentioned packets.
Sorry to post to an old topic, but anyone find anything out about this? I am also interested in using wifi in promiscuois mode to delve into raw packet data.