BillyS, a senior member at WebmasterWorld posted that he thinks Yahoo Search might be updating now. BillyS is known for watching Yahoo Search pretty closely, but in this case, no one has come in to back his observation.
Anyone else seeing some pretty big shifts on Yahoo? We normally get around 12% of our traffic from Y!, but today that value is closer to 18%.
I don't expect that value to stick, but I'm expecting some settling over the next several days.
I personally checked some of my traffic from Yahoo on this and some other sites and they don't seem to have jumped up or down that much. But I do not track Yahoo as closely as other SEOs.
The last observed but not confirmed Yahoo update was towards the end of March 2009.
Forum discussion at WebmasterWorld.
Update: This is now official, Yahoo posted about this "weather report" at the Yahoo Search Blog on June 2nd.