Yahoo is in the midst of testing a new algo and filters. I see even more strange results if I check SERP's in the middle of the night. FWIW, it looks like they are experimenting with some very aggressive filters - most of which are garbage.
In fact, I think that they have found a way to penalize 50% of the good sites out there and replace them with spam
I think that this Yahoo update will be what the Florida update was to Google. But with Yahoo being notoriously slow to accomplish anything, my bet is that we won't see the final results for another 3-4 weeks.