We've had rumors that Google might introduce a system to restrict how many affiliates can bid on a term on behalf of a particular web site. Now one of our forum posters is providing further confirmation this will happen.
See our One Customer Per Landing Page thread where our forum moderator AussieWebmaster gave some confirmation in December of what he's heard on this, with other members contributing. AdWords - 1 affiliate per merchant to be announced tomorrow is a fresh thread with the news it may come this week.
For some background on why Google might make such a move, also see: