Yahoo doesn't link directly to your site (anymore?). It uses a redirect script which lies on a different domain (rds.yahoo.com). This domain is forbidden to crawl via robots.txt exclusion, and therefore the link will be invisible to google and all other search engines.
There are also local Yahoo, like asia.yahoo.com, uk.yahoo.com, au.yahoo.com etc... Those link directly. If you are accepted in the main Yahoo directory, your site will automatically appear in all local Yahoo's as well, and those have the same importance and do show as a backlink, but the main Yahoo directory doesn't.
Though UK Yahoo has not been updated for quite a while, at least for my category.
I looked into this quite a while ago, when Yahoo still had a business relationship with Google. Even then there was a redirect through that subdomain that was theoretically off-limits to spiders, but Google had the backlinks anyway - either by doing the spidering anyways by special permission, or maybe by simply stripping out the redirect portion of the URL.