Social Media and Law Enforcement

What are the LIMITS to how social media is used?  What safety concerns can arise over taking pictures of an arrest as it is happening?

Charlotte Observer:
http://www.charlotteobserver.com/2011/07/03/2425557/recording-police-can-create-clashes.html

Poynter:
http://www.poynter.org/latest-news/media-lab/social-media/134489/with-social-media-police-and-reporters-grapple-over-control-of-message/

http://www.poynter.org/latest-news/top-stories/134785/when-crime-victims-tweet-new-and-old-dilemmas-meet-for-news-organizations/