I'm not a lawyer, but: the cops can still hassle you even if you know you're in the right.

Consider this case: http://articles.baltimoresun.com/2010-05-08/news/bs-md-herma... Eventually a judge ruled for him, but imagine the heartburn over the prospect of going to jail for 15 years. Some cops do not like being video-taped even in public. Some will even pull a gun on you to intimidate you: http://www.miamiherald.com/2011/06/02/v-fullstory/2248396/wi...

I read a case that if you look like you're recording everything in general, the people being recorded give permission to be recorded in public space.

I think it was aimed at protecting journalists though.

Just depends. The cops will still screw with you, hence, "You can beat the rap but you can't be the ride."