The controversial NebuAd technology has been a source of concern for privacy advocates, following numerous ISPs partnering with them. The company specializes in web-based tracking of user habits, with the goal of delivering targeted ads. The problems were twofold: People saw it as an invasion of privacy, and existing advertising companies/website owners saw it as theft, since ISPs could prevent users from seeing the ads the site owners wanted.
Even if it seemed like a good idea on paper to the ISPs, in practice it isn't going over well. Fearing an “unfavorable climate”, among other reasons, numerous companies have decided to drop their plans to incorporate the NebuAd system.
Some of the companies that decided against it did so due to fears that it is a legal grey area and don't want to risk getting in trouble. Others tested the program, and for whatever reason found it lacking. Still others do see it as an invasion of privacy. NebuAd hasn't commented on the numerous dropouts, including whether or not it is a setback to them as a company. I can't say I feel bad for them. Whether or not they can "personally identify" you, it doesn't seem right to have the ISP - or anyone - interfere with the content at the sites you visit.