Oh one thing that's not mentioned, Burning man regularly sees attendance over 50,000 and runs for a full week.
Aerial view of Burning Man:
http://www.geeked.info/wp-content/uploads/2011/09/Burning_Man_BRD_NV_GE1_1SEP2011_600x600.jpg
EDIT: here it is in Google Maps so you can zoom in.
https://maps.google.com/maps?q=burning+man,+NV&hl=en&ll=40.783401,-119.20372&spn=0.040878,0.071754&sll=37.793101,-122.39645&sspn=0.085325,0.143509&t=h&hnear=Black+Rock+City,+Pershing,+Nevada&z=14
EDIT2: If you want to get an idea of how effective their zero trace policy is, they move the location of the "city" every year by at least its full footprint. Zoom around that google map and you won't see the previous footprints.