It has been WebmasterWorld about a possible update that started rolling out on December 1st.

Some reported seeing an update and then said Yahoo seemed to have rolled it back. But now more are reporting seeing significant changes in the Yahoo Search results.

Of course those who are negatively impacted are the loudest:

Observing lots of changes in Yahoo

Rankings have started to disappear :(

We currently do not have a "weather report" from the Yahoo Search Blog announcing the update. But typically, we beat out Yahoo posting such news by a day or two.

Forum discussion at WebmasterWorld.