The telephone example is near exactly Google Voice, right? Massive voice database for the cost of some subsidized VoIP and storage?

"...cost of recording, storing, parsing, and analyzing all that data..."

You do realize that this is exactly the Google and Facebook business model, right? Targeted advertising using user-generated content?

"The number of people that might sign up for privacy invasion in the interest of a 2 for 1 Big Mac would be so insignificant"

Over a billion people have signed up for Facebook, giving up their own social graph data and (as we've seen recently) address books, and that's just for a free shitty profile page whose design changes on a whim. Add a burger to that, and you've got a deal.

There's no slippery-slope here--at this point, the dataset is large and obvious enough that if you aren't blinded by the magical nerd future you'll see very reasonable concerns over what might happen.