Behavioral targeting is a tough market currently, as more and more web users (and regulators) become aware of that their surfing leaves a wake that can be followed by advertisers. NebuAd is one of the few, along with Phorm, that are pushing even more aggressively, using information purchased from ISPs. Older behavioral targeting companies only know when a user visits a site within their network, but ISPs can deliver information on every site a user has visited. Now the company is being investigated by Congress, under federal wire tapping laws, about whether it's gone too far.
Regulation is clearly needed, but behavioral targeting is here to stay. The irony is that ultimately this sort of tech is needed by every part of the online advertising equation. Publishers want to get higher CPMs, which comes with targeting relevant users. Online media buyers absolutely need to be able to guarantee that the people viewing the ads are those that could conceivably be interested in the product. And consumers, who are understandably squicked out by overly-invasive tracking, are still better served by seeing ads for products they want while they browse the sites that are running because online advertisers pay the bills.