The Yahoo Search Blog has posted that they are currently undergoing an index, crawl, and algorithm update. Priyank Garg of Yahoo said:
Over the last few days, we’ve been rolling out some changes to our crawling, indexing and ranking algorithms. While we expect the update will be completed soon, as you know, throughout this process you may see some ranking changes and page shuffling in the index.
I posted signs of this update this morning at the Search Engine Roundtable, and linked to several forums discussion, which highlight what appears to be a pretty significant update.