Advertising companies also have legitimate needs for tracking exposure to advertising across applications. Frequency caps, for instance -- advertisers buy inve

Advertising companies also have legitimate needs for tracking exposure to advertising across applications. Frequency caps, for instance -- advertisers buy inventory across multiple applications with the explicit provision that a user won't be shown the same ad creative more than a specified number of times in a given period. Not having cookies accessible across applications is the true design flaw. No one particularly wants to use the UDID for anything. Everyone would much rather use exactly the same infrastructure used for web advertising.