In iOS 6, under the Settings>General>About section, you will find an item near the bottom called Advertising. When you touch Advertising, you will see an option of turn Limit Ad Tracking ON or OFF. You can also Reset the Advertising Identifier. At the bottom of the screen you will see a link to Learn More. This note tells us that the Advertising Identifier is a device identifier “that apps will use to give you more control over advertisers’ ability to use tracking methods…. In the future all apps will be required to use the Advertising Identifier. However, until then you may still receive targeted ads.”
It appears to me that this is not a fully implemented feature, but they have added the edit option to be prepared for the future. If you are receiving numerous targeting ads in apps, you may wish to try turning ON the Limit Ad Tracking feature. I believe this is the default setting.