Think about that for a minute. Again. The amount of advertising space is NOT going to DOUBLE just because Yahoo introduces a contextual ad program. Websites will have EITHER Adsense or Yahoo, not both at the same time.
Let's assume that 100 percent of website publishers out there now have signed up for Adsense. Those that don't stay with Adsense will go to Yahoo, but advertising space with not actually double. It can only double if there are twice as many publishers getting involved with either program.
Advertising prices may drop because of increased competition as advertisers shop around, but that will be a pain for many of them to keep up with all the time. Sooner or later, CPC will even out across the board at both places when things settle down.
My guess is Yahoo will offer advantages that Google doesn't and vice versa, and publishers will have to choose which program has the advantages they prefer.
The nice part about a new contextual program is that when I get kicked out of Google (for no fault of my own), I have someplace else I can go. This offers more stability to me as a publisher.
I don't mind losing a few dollars per CPM if I get some more stability. Afterall, I still want to be doing this 10 years from now.