> and I can't see any site I've visited today that appears to be funded only by the kind of targeted ads we're discussing.

I think thats a key point. ALL banner advertising would be affected by this change. All ad servers use cross domain cookie tracking of some kind. If not to track user behaviour, its (for example) to track impressions to make sure that you dont show the same ad to the same user more than a set number of times. That is also a 3rd party cookie.

Without that ability it will be hard (impossible?) to rate limit campaigns to users, thus increasing the cost effectiveness to the advertiser and eventually hurting publishers as advertising money is diverted elsewhere.

In short, this rule would affect ALL banner adverts on all sites. So 99%, yes (at least the content sites I have read today).