Jeff Jarvis shines a spotlight on a webroach called Vibrant Media. They're pushing something called IntelliTXT, which will scan content for keywords related to those of advertiser's products, and intrusively add direct hyperlinks to the scanned content. I can easily see freeware bloghosting sites adopting this monstrosity, and making their bloggers into inadvertent shills.
Google's targeted ads don't bother me - they stay in their place, they're clearly labeled as ads, and it's well understood that the writer has no control over them, except in the indirect sense of talking about X will elicit random ads on approximately the same subject. I've seen forums (such as Angels from Another Pin's, ferinstance) amuse themselves by attempting to engineer the Google ads with targeted jabber.
This "IntelliTXT" goes beyond mere targeting, and literally hijacks the content. If I'm ranting about the horrors of the Dean candidacy, I don't want to spasmodically link to Margaret Cho's site without actually, you know, intending it. I could imagine a MoveOn enthusiast ranting about Bush and Hitler, and find herself accidentally linking to David Duke propaganda, or, even worse – Kim du Toit. Neither outcome is something we objectively want to see, however funny it might be in the heat of the moment.