Google changed its policy regarding affiliates and their ability to market via Google AdWords.
Beginning later this month, only a single ad from a single affiliate will be visible for a product or service alongside a company's own advertising. This will be based on the affiliates Ad Rank. Google said that it will "improve relevance".
While I might agree with their point concerning relevance, it more grossly effects 1) the revenues of top affiliates and ; 2) the companies who rely on those affiliates to generate sales.
It was affiliate marketing guru Beth Kirsch that brought this to my attention. According to Beth, it's possible for a top affiliate to generate millions of dollars in revenue for the companies they represent - and possibly create millions for themselves - through arbitrage. Read her take on it here.
It's going to be interesting to see if companies go to the mat for their affiliates and lobby Google on their behalf. (Which might prove fruitless, anyway...)