Via an email to affiliates, Amazon.com announced they are scaling back their associates program in North America by disallowing direct linking from paid search results:
After careful review of how we are investing our advertising resources, we have made the decision to no longer pay referral fees to Associates who send users to www.amazon.com, www.amazon.ca, or www.endless.com through keyword bidding and other paid search on Google, Yahoo, MSN, and other search engines, and their extended search networks. If you’re not sure if this change affects you, please visit this page for FAQs.
As of May 1, 2009, Associates will not be paid referral fees for paid search traffic. Also, in connection with this change, as of May 1, 2009, Amazon will no longer make data feeds available to Associates for the purpose of sending users to the Amazon websites in the US or Canada via paid search.
As the paid search market has matured and competitive research tools have improved the value of using affiliates for discovering new keywords has been sharply reduced. Other merchants will follow Amazon’s lead. Some might wait for the economy to pick up first, but the fact that Amazon is trimming this in a down market (recession/possible depression) shows how little they feel they benefit from affiliate arbitrage of paid search results.
Yet another reason to find SEO based affiliate work more stable than PPC affiliate efforts. Any PPC-based affiliate that is sick of having to rebuild from the ground up over and over again would be smart to (and is welcome to) taking the enlightened path 
