iOS 6 User Tracking feature
The Identification For Advertisers (IFA) feature present with Apple's recently released iOS 6 is enabled by default and allows for user tracking! IFA is a random number generated once by the iOS device which is used to uniquely identify that device between applications. The number is available to apps which can send it to their advertising service of choice to pull down new adverts.
Read the full article here - http://www.theregister.co.uk/2012/10/17/itrack/