[dpeck]

Mid summer tends to be pullout season for Blackhat and Defcon speakers. A handful happen every year, thats why they have alternates.

Sometimes the speakers screwed up and didn't get their material together and they weren't important enough to ignore that. Other times they're threatened by their employer or some external forces.

Subway hacking, Padgets RFID (and GSM a few years later IIRC), etc. Theres quite a history of great presentations that have never happened for one reason or another.