Ethics in Photography

I was browsing some photographs on Flickr when I learnt about Jill Greenberg's controversial photo exhibits of crying children.

Jill Greenberg is a Sick Woman Who Should Be Arrested and Charged With Child Abuse
Think In Pictures: Adventures in Visual Education: The Case Against Jill Greenberg's "End Times"

I guess there are plenty of grey areas even in the work of a photographer ...

• Shooting Strangers: Where do we draw the line of invading someone's privacy in a public place?
  
• Shooting Strangers' Kids: How can a parent not be paranoid about potential kidnappers (or worse) and their M.O. as casual photographers?
  
• Permissions to shoot: Do we always have to respect the "No Pictures Please" signs posted?
  
• Security: If enough of us take enough pictures of the Brooklyn Bridge, are we helping terrorists?
• Dignity: Can we call ourselves journalists just because we happened to have the opportunity to witness someone's vulnerable / embarassing / unfortunate situation while carrying a camera?
• Skin: How much skin is too much?
  
• Taboo: Photographers love taking pictures in avenues that challenge society's norms and conventional thinking. But is Jill Greenberg's work ethical?