A thread at WebmasterWorld shows a possible Yahoo! index or ranking update taking place now. It may be the very early stages or it may not be happening at all. But confirmation comes from Dayo_UK, a senior member at WebmasterWorld, "I now see this update starting." The update seems to be coming in and out, some are noticing it, some notice it and can not see it now and so on. Very "Google Dance" like, are how people are describing this update.
I will track this and provide updates as it happens.
Forum discussion at WebmasterWorld.