Yahoo! rolled out some changes to its search index with "fresh web data" and updates to their crawling, indexing, and ranking algorithms over the last three to five days.
Yahoo has had just two updates since last November: one in December, 2008, and another in late January this year. Yahoo! expects the update will be completed very soon.
As with any index update, you may see some changes in ranking as well as some shuffling of the pages in the index.
Are you seeing some improvements on the SERPs for your website?