I don't see a directory listing be a magic ingredient .. one of my clients sites has been seriously demoted in the serps and they have a directory listing yet I've seen a few site gain without a Yahoo Directory listing....
Remember that Y! don't have all the cool G semantic tech yet. So for sites to really benefit they need to have their category closely matched to the target search phrase. Without a close match you can expect a smaller increase in rank. Also because Y! use category descriptions you could actually be handicapped if you have a bad description.
So if your site about ‘widgets’ is in the ‘widgets’ Y! category then your doing well for a search on "widgets"
However, if you target ‘online widgets’ then don't expect the same boost - especially if you don't have 'online' in your description.