When it comes to Yahoo! conducting an update, we normally certainly know about them when we have a "weather report" from the Yahoo! Search blog. But sometimes, like with Google, but on a much less pronounced level, forums pick up on index and algorithm shifts prior to the update.
A WebmasterWorld thread suggests that the beginnings of a new update is happening over at Yahoo! Search.
One member sees a big jump on the Yahoo.com search results. Others do not necessarily see the changes yet but suspect something is coming soon.
Forum discussion at WebmasterWorld.