It's plainly posted, right behind the sign that says "Beware of the 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.
For my sins I did a stint in the mobile ad space. One of the issues is that various parties wanted a way to uniquely identify a user, and failing that settled for uniquely identifying a device. This is because multiple different parties can report against the same one. For example say you publish an app and buy ads on networks A and B. A & B play the ads and then notice the app has been installed. They both come back to you and say "we played your ads, the user installed the app, pay up". Without some way you can identify the user/device and they identify the same user/device you cannot check what actually happened. Similarly if ad network A is present in two different apps they don't want to overplay the same ad at you. And if you respond to a car ad in one app, chances are better you'll respond to a car ad in the second app.
On iOS the identifier used was the UDID which is a permanent unique identifier the device. This remains even if you wipe the phone and give it to someone else. It also makes it easy to track users because you can correlate activity across different apps and ads by using the common id. Apple started getting antsy about this in the iOS 5 time frame, so a bunch of ad folk got together and made OpenUDID. The intention there is a unique random id is generated and then multiple apps cooperate on using it. Pretty much the same tracking issues, but the value can be wiped at any time.
With iOS 6 Apple have now taken control of this unique id. It is randomly generated and is wiped when the device is reset. And when you turn that track limiting on the unique id is not available.
Various other folks try to get around the UDID issues by finding other unique identifiers such as phone numbers, MAC addresses (present for wifi and bluetooth) etc. On Android the READ_PHONE_STATE permission lets you get the telephony ids, and increasingly users are punishing apps for requiring it in Play store reviews. (One flaw is that you can't do a review unless you install the app though!) An article from the Android side.
TLDR: Evil advertisers need to track your activity across different apps on the same device. With Apple's help you can now have the illusion of sticking it to them.
So there's a looming "buy a used phone, get somebody else's phone-tracking ads" issue analogous to the "buy a used set of eyes, get somebody else's retina-scanning ads" scenario?
Thing is, this is designed precisely to avoid that scenario.
I'm pretty sure that's the toggle for "advertisingTrackingEnabled" etc in the new "ASIdentifierManager" class.
https://developer.apple.com/library/ios/#documentation/AdSupport/Reference/ASIdentifierManager_Ref/ASIdentifierManager.html
Btw, I thought reading MAC addresses were grounds for app store rejection, just like the old UDID uniqueIdentifier call.