The last 2 days are serps have went crazy on both Yahoo and Bing, to the point where we've all but disappeared in both today. I wonder if they're starting to replace Yahoo with Bing results now? I posted a question on the feedback forum for Yahoo site explorer asking them about it, still waiting on a non-ambiguous answer. I wonder if site explorer will be replaced by stupid microsoft webmaster tools when the merge is complete?
I have the SERPs crazy too especially on Bing. In my opinion bing will use some of yahoo algorithm to suit EU users.
Bing will use Yahoo search databases, not the other way around. Bing is really targeting shoppers. It is not trying to be "just another search engine" they want to be a shopping decision engine.