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It's buried at the bottom of the page, but they link to the "Keep My Opt-Outs plugin".

Just curious: are you opting out? Why?

How else could it be done?

You'd need some kind of browser extension (aha!) to purposely keep that placeholder cookie set. Then you've just traded one problem for another.

Now, if the browser came with it, and defaulted to on, then we'd really have something to talk about.

On the server side. Associated with your account.

With what account? Google serves on a ton of third party sites (and not just through AdSense, DoubleClick is huge). And that would require you to sign in to Google before you are opted-out, regardless if you were using Google or not. I use an plugin provided by Google to do the same thing, no worries about a cookie disappearing.

http://www.google.com/ads/preferences/html/intl/en/plugin/

An HTTP header.

if it's any help, i use ghostery and disconnect plugins (are they called extensions?) in chrome and that page lists nothing for me. so one way to opt out permanently is to use those.