Those who make their money by working out who sent you to visit which websites via what adverts are scratching their heads and popping the Advil this weekend over what to do about Mobile Safari, which automatically blocks third-party cookies by default.

This makes it difficult for ad servers, tracking systems, and ad management tools to link visitors to ads that brought them to the Web site. Which, in turn, makes it difficult to measure the performance of paid-search marketing campaigns.

Search firm Marin Software published a white paper last week which says that Mobile Safari on iOS devices is a "major challenge" and that, on average, advertisers using third-party cookie-based tracking systems are undercounting conversions by 38% -- the actual conversion rates for iOS, minus for the third-party cookie based undercounting, were on average 23% higher than on Windows.

With millions already using iOS devices and the iPad 2 and, later this year, a new iPhone bringing millions more into the Apple fold, this is becoming a big problem for ad companies.