WWDC 2012 sold out in a matter of hours. San Francisco has lots of privately owned public open spaces during business hours. WWDC Spaces is a place for us to organize a list of good ones near Moscone so developers looking to escape Moscone and non-attendees alike can gather. Leave a comment here and I'll add them to the wiki with links.
SPUR has a list of public spaces downtown. Have you been to one? Leave a comment with info about it! (I can’t visit all of these, though I’m going to try.)