Last year Google released a major update to its index right before Christmas, and all hell broke loose. This year Yahoo did the same, but the reaction has been more muted - even positive, in some cases. It's always hard to draw conclusions from index updates, but this one seems to be moving a lot of SEO links around the SERPs...

Update: According to SEORountable, an update is happening over at Google as well, though opinions are mixed as to whether it's major, or simply ongoing maintenance...