When I get time.

No Ads for Regulars at Binary Bonsai
I think I originally got this code off of Matt, and now here it is for you to use. All this does, is check to see if the current page is a permalinked entry and if the person visiting has a cookie from leaving a comment.

If it is a permalink and the user doesn’t have a cookie, they get an ad.

global $cookiehash;
if ((is_single()) and (!isset($_COOKIE['comment_author_'.$cookiehash]))) {
Google Ads can go here…
}

It requires WordPress of course, but other than that it’s easy peasy.