SEORoundTable points to an interesting thread at WebmasterWorld which asks how Yahoo! ranks sites within its directory.

The answer? According to Yahoo:
By default, Directory site listings are presented sorted by popularity and relevance. Sites that are most popular with users or the most relevant to the category appear at the top of the site listings. The order of web sites or web documents is based upon Yahoo! Search Technology. If the category is large, the listings will display over multiple pages with pagination displayed at bottom of the page.

Interesting stuff. If you've got a listing in the Yahoo! Directory and you're looking for better placement, build more links and the Yahoo! Search Technology will do the rest!