Based on the thread it looks like most won't be loyal and will try out Yahoo!'s AdSense-like product when it comes out. The serious issue is that Google does not allow one to run competing contextual ad programs on the same site that Google AdSense is running. So publishers will have to be creative and do time based testing to not run Google Ads and Yahoo! Ads at the same time.
I might personally make a request to run the ads side by side. I would love to not only test CTR, CPC, revenue earned but more importantly, perceived relevance with these contextual ad competitors. How cool would that be?