Don't think it's likely. Yahoo might have already reached it peaked, and it's probably overvalued. I'm not sure what Yahoo has that Apple could possibly want for $20+ billion.
Apple vs Google+World
Apple's hardware can push ads through iAd to make more money. But with Yahoo's property as an extra, iAd all of a sudden has enough advertising value to justify itself. Without Yahoo properties, iAd is a much smaller network.