I don't think it will substantially increase the number of publishers. It might go up some, but not a lot. I think the cream of the publisher crop has already become Adsense publishers. Some will go over to Yahoo for the diversification, but only when new publishers sign up is advertising space going up.
I think Adsense already has the best of the internet because look at how they are asking people to recommend a friend to the program. Google is also having their employees "cold call" non-AS website publishers as was related in another thread.