I'm interested also -- I live in Rockville and work in McLean..
Only problem is that I don't have a gps unit and even if I did, I don't think macstumbler works with gps yet.
However, I do have a super k-rad directional soup-can antenna that I can twist around really fast with my ninja skillz, for that extra home-style dBi.
I'll still come out just to have some fun. Maybe I can organize the post-stumbling party.
-Cpt'n
EDIT:
Hummm.. I think I may have a solution for myself:
Yellow Dog linux
Orinoco driver patch
Kismet
USB -> Serial cable
Garmin GPS unit