I reside in Canada therefore not able to sign up for the Yahoo Publisher, since they are still in Beta Testing. One reason i believe Yahoo publisher is a serious potential competitor, once the service goes worldwide, is because the minimum send-out amount is $50, as compared to Adsense's $100! Also a question: Google has human editors to approve the websites and ads of the advertisers (AdWords). Yahoo has human editors to approve websites of the publishers. Who is right? And who is better?
I think that Yahoo! is going to be the better service here. I still have not had a chance to try Yahoo! because they keep turning me down, but I'm definately wanting to try them out. The reason I think Yahoo! will be better is because as an advertiser I know what I want to advertise, but I don't know what kinds of sites my ads will appear on. So having a human check the sites my ads appear on will be good.
google has been playing the adsense game for some time now. I really dont think they need to worry about competition. If the competition does start to heat up, they will just compete back.