I suspect Embarq is one ISP that is contracting with Nebuad. Here's a Wall Street Journal article that seems to imply it:
»online.wsj.com/public/article_pr ··· 192.html
The relevant passage is:
"Both wireless and wireline Internet-access providers such as CenturyTel, Rochester Telecom Systems Inc. and Embarq Communications Inc., among others, have entered the advertising gold rush. And they've tapped Internet equipment companies like NebuAd, FrontPorch Inc., and Phorm Inc. to provide the gear to help them along."
"Preference Advertising. EMBARQ may use information such as the websites you visit or online searches that you conduct to deliver or facilitate the delivery of targeted advertisements. The delivery of these advertisements will be based on anonymous surfing behavior and will not include users' names, email addresses, telephone numbers, or any other Personally Identifiable Information.
You may choose to opt out of this preference advertising service. By opting out, you will continue to receive advertisements as normal; but these advertisements will be less relevant and less useful to you. If you would like to opt out, click here."
Opting out sets a cookie, so if you delete cookies periodically or use a different computer, you have to do this everytime.