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Just saw this today: [out-law.com...] - this seems to have slipped under the radar a bit with the new EU law designed to hit file sharers.
Basically, user consent is going to be required to send cookies to a user - except in the case where "strictly necessary for the provision of a service explicitly requested by the user – so cookies can take a user from a product page to a checkout without the need for consent".
so Adwords conversion tracking, Analytics conversion tracking etc is going to get a whole lot more difficult as this isn't requested by the user...
Cookies are also destroyed if a person has privacy enabled in their browsers. Most newer browsers offer this feature.
Any good web developer knows that storing important information in a cookie isn't an effective way to track that information.