What the people nearby are saying is incidental to the thing you're actually recording, though. It's the same thing as, or very similar to taking a video while you're on vacation: the police aren't going to bust down your door and haul you away because your camcorder's mic happened to pick up a conversation between the people standing three feet away from you. When you break out a parabolic mic and start recording conversations from 30 meters away — without the knowledge or consent of the people having that conversation — however, you're doing something that's at best creepy, and probably also illegal.
EDIT: It would also probably be illegal to, in the course of taking your vacation video of whales breaching or whatever, instead start specifically recording a conversation happening nearby. IANAL, but I think the distinction has more to do with what you intend to record than what you happen to record. You don't have a "reasonable expectation of privacy" in public, but you do have a "reasonable expectation" of not being deliberately and specifically recorded by some random creeper while out in public.
EDIT 2: Further, with as often as it happens, and pursuant to the DoJ's own statements on the subject, the police don't have a "reasonable expectation" of not being recorded when they're going about their jobs — particularly when, in the course of executing said job, they end up beating someone into unconsciousness...
Personally, I feel such matters are resolved by the simple rule that things happening in public spaces are public, and things happening in private spaces are private. One can argue about public vs private spaces, but it removes intent and expectations from the equation.
Are you outside your house? If yes, expect to have all your actions recorded and available to be streamed on the internet.
Your recording is also likely to pick up the conversation of the people nearby saying "hey look at that".