That would make a lot of sense: Let Yahoo come into the marketplace with trumpet fanfares, then announce "Um...we've got something better" and roll out AdSense: The Sequel.
Oh absolutely. Look at everything that's been announced by any of the search engines lately. It's always a release and press release war among the big three now. Yahoo couldn't announce new toilet paper in the men's bathroom without Google or MSN trying to one up them with a "Two-ply and Softer!" press release the same day or next day and vice versa.