This essay by Josh Chasin over at the MediaPost's Metrics Insider Blog is the best piece I've read on behavioral marketing & privacy in a long time. I like this analogy, in particular:

Let's say you are a tall, dashing, smartly dressed Chief Research Officer at a major Internet audience measurement company, and you walk into Nordstrom's. A sales clerk you recognize comes up to you and says, "Hey, your wife's birthday is coming up in a few weeks, and we just got in those sweaters she likes. Should I put a couple of them away for you in her size and color?" Now let me ask you. Does this hypothetical Chief Research Officer perceive this to be: (a) an egregious violation of his privacy, causing him to immediately rush home and write his state assemblyman; or (b) another example of Nordstrom's world-class customer service? If you answered (b), then you're tracking with me so far.

So how come if this exact same thing happens on the other side of the screen, it stops being outstanding customer service and turns into a violation of privacy?

Great question! And yet some over-zealous privacy advocates make this stuff out to be the coming endtimes and call for comprehensive regulation using scare tactics and twisted logic, as Chasin notes:

If Big Brother barges into your home at midnight and takes you away because someone doesn't like the books you've been reading, that's an invasion of your privacy (and way worse.) But if the ads you see on Yahoo are increasingly relevant to your life, that's not an invasion of privacy. That's just the digital version of that nice lady at Nordstrom's. Let's not confuse the two.

Exactly.