[amluto]

That would partially defeat the purpose. First party adds see first party cookies, so they have no inherent cross site tracking ability.

[asiachick]

And yet facebook is already do something along the same lines by adding fbcid=<trackingnumber> to every outbound link and then the site that receives the link can report back "I saw fbcid=<trackingnumber>". Sure it makes 3rd party tracking require more trust but wouldn't some analysis tell you if your client is trying to game your ads for revenue etc...?

[manigandham]

Cross-site tracking via cookies (and 3rd-party cookies in general) has already been dead for years.