Here and there on the web are articles about Identifier for Advertisers (IFA), introduced on iOS6. How can it be accessed from WebApplication (i.e. from Javascript that runs in the mobile browser)?
Actually any well-working/well-formed way to track user context for ads relevance will be mostly welcomed.
According to this post the Ad-ID is not accessible in the browser:
http://blog.adfonic.com/advertisers-2/ios-6-for-mobile-advertising-part-one-smarter-identification-through-ifa/