If they start poisoning search with for-profit results Google will be quickly reminded that they are not the only search engine in town.
As long as the ads are marked somehow, a user script will be able to suppress them.
It's sad I need to mangle the web to make it usable, but not as sad as not having a mangling facility would be.
As someone who runs NoScript, Adblock Plus and various other user scripts, I think it's a good thing to be able to take control over your own experience. It's good not to be passive. It's good to see only what you want to see. The more people do this, the more these companies have to comprehend that this is the nature of the network in which they have chosen to participate. The Web would lose most of its appeal to me if it were entirely corporate controlled like television.
What's really sad is that other mediums like television, radio, and periodicals are one-to-many, heavily centralized, and don't include such great control over what you see. They're package deals that require you to accept the crap along with the content and are often not worthwhile.