Originally Posted by torka
Google (and other reputable search engines) have been quite clear about this from the very beginning.
Not only would it be a bad idea from a business standpoint, in the USA (and many other parts of the world) it would be potentially illegal.
Although I agree with this conceptually, as documented in the recently leaked FTC investigation report of Google, there have been pay for ranking schemes where Google has manually manipulated the SERP results if a company did not provide something to Google.
Much depends on whether you are going to accept Google's public statements, or not. Based on the FTC report I wouldn't.
Google has a financial incentive to downgrade the ranking of a website that could, but doesn't, pay for PPC. Even the subtlest of downgrades - one spot - can make a huge difference on your bottom line.