Jane McGonigal gave a wonderful keynote on games and ARGs. Not dissimilar to her ‘Reality is Open’ presentation at GDC recently, so these are not her SXSW slides, but there’s an overlap.
Other people have made excellent notes already, which are much more complete than my own. Here are some:
During the keynote, Jane mentioned that as part of playing an ARG, she had witnessed (and learned) the Soulja Boy dance, and offered to perform it at the end of the session. She kept her word.