This is the official policy: Don't take pictures of anyone unless you have permission from them. This includes panelists, honored guests, and regular congoers. It includes parties, hallways, and the lobby. If you want to take pictures of a panel, arrive early and ask before they start; most people will be okay with this. If you want to take pictures of people at parties, just ask them; again, most people are generally okay with it.
As for blogging and posting, do not post pictures you've taken unless you have permission from the people involved. If you want to blog about panels without using pictures, that's perfectly fine to do; anyone who wouldn't want their name mentioned will be using a pseudonym or fan name at the con.
This policy is to protect our members from harassment and possible real-life consequences for their careers or other issues. Thanks.