Y! is in the midst of upgrading their algorithm. Many are noticing their keywords positions jumping all over the place. The storm will last approximately 2 weeks according to the blog entry. The search engine was offline for a brief period.
Yahoo! still has about 30% of the marketshare, according to Nielsen/Netratings. I think it's smart to keep them in mind as part of a marketing mix. I also find that it's easier to rank well on competitive terms, as it is a less competitive marketplace.