It's plainly posted, right behind the sign that says "Beware of the
Leopard Mountain Lion":
Settings → General → About → Advertising → Limit Ad Tracking → On
It's far from clear what, if anything, this actually accomplishes, but it's a thing that exists.
Update: Oh, hey, here's another:
Settings → Privacy → Location Services → System Services → Location-based iAds → Off
This is a level of obscurity worthy of Facebook. Well played, Apple.