I found this handy PDF while surfing around last night. It describes the rights a photographer has to take pictures out in the Real World. All too commonly, even the police don't know the law.
In short, in most cases, "If you can see it, you can photograph it". This rapidly approaches 100% if you are on public land. It is perfectly legal to take pictures of private land FROM public land. Under no circumstances is it legal for a private entity to demand your equipment or film.
If you take pictures out on the streets, print that PDF and keep it with you.
Ooooh. Thank you for linking to that. It answers some of the questions / concerns I have about taking pictures around Boston (especially given the harassment that people get in South Station for taking pictures).