In a recent thread titled, "December 1, 2007: Yahoo Update?" at WebmasterWorld, there are assumptions made my users for a possible update coming out on December 1st.
Some even reported of noticing an update, while others left that Yahoo! rolled it back this time. However, many are reporting of significant changes in the Yahoo! Search results.
One user said "Anyone else seeing changes today? No weather report on the yahoo search blog yet, but I just noticed a big jump in 2 of my sites today." There is a another user saying, "Observing lots of changes in Yahoo. Rankings have started to disappear."
Till now there is no "weather report" from the Yahoo! Search Blog. Instead of that it has a blog titled, "Top Trends in Search in 2007" which highlights the changing trends in searching in the current year.
Forum discussion at WebmasterWorld.