While I was walking around the Commons, shooting a few photos, a couple of security guards told me (nicely) that photography was not permitted. I had already taken the photos I wanted, and they didn't ask for the camera.
Only later did I see and photograph the rules. (Click to enlarge.) While photography isn't explicitly banned, it apparently qualifies as "unauthorized activity."
[A commenter on Gowanus Lounge suggests that the ban is related to the presence of city emergency services. Maybe, but I was nowhere near those buildings.]
That raises a question. Should Atlantic Yards open space be built--the eight acres would appear in the increasingly distant second phase of the project--would photography be permitted?
The open space would be managed by a conservancy or nonprofit agency, not the developer. Surely they'd be hearing from the Atlantic Yards Camera Club.