I believe that press (i.e. people taking pictures for publication) have to register at the Info Desk, get their press ribbon and then do the "get permission" thing from those they are photographing.

Others are pretty much free to take pictures in public areas (e.g. not in the Art Show or during the Masquerade). As others have said, asking permission first and accepting "no," if given, is a very courteous thing to do and courtesy is good.

Here are the official policies (http://2006.arisia.org/index.php?option=com_content&task=view&id=92&Itemid=109); note the section at the end.