There are some early reports of a Yahoo! Search update or algorithmic change. A WebmasterWorld and DigitalPoint Forums thread have reports of shifts taking place. Of course, you see that those posting, currently at least, are unhappy with the result sets.
To me the results seem fairly static from where they were last week. But that is only for the few keyword phrases I looked at.
There are no "weather reports" posted at the Yahoo! Search Blog.