A nice article on the legality of photography these days, via Consumerist. Wired also has a piece this month on the topic, by Clive Thompson. Intellectual property laws being what they are today, it's important to know what the law actually is (although it differs from country to country and even from state to state). But for the most part, common sense prevails - in the public domain, for non-commercial purposes, you can photograph just about anything without permission, as long as it's not a security risk. Read the article, though, and/or consult your lawyer if you plan on making use of this information, though. Damned if you're going to try to take me down with you, paparazzi pond-scum!
p.s. I'm being generous - pond-scum is the nicest kind of scum.