A WebmasterWorld thread is reporting a large change in SERPs at Yahoo! I have noticed this myself. On the second page message number 19 by BillyS, he reports "I've lost about 50% of my pages after growing steadily since April 1st. In fact, the results look like they are based on an index pre April 1st." I have noticed exactly this, but let me be clear, I am not tracking and watching Yahoo! like a hawk. I have noticed this on the keywords I happen to watch at Yahoo!.
I tend to believe that this is not the real update and that we can expect something larger in the next two to three weeks. But I can be wrong.