Charter's Trials of NebuAd Halted
It isn't a "would be" implemented. It's already done. Basically, there is no way to prevent your ISP from altering your traffic, because everything that comes over the wire passes through them. You have no way of telling whether the ads served on a website are the ads that that website sold or whether the ISP inserted them without controlling both ends of the communication.
Dan Kaminsky developed a method to detect this kind of tampering , which at least can prevent ISPs from hiding the fact that they're doing this. It's in the second half of this talk, from last year: http://events.ccc.de/congress/2007/Fahrplan/events/2393.en.html