marc -
I agree with Bill - and when I asked the rep from Yahoo - he gave me the stock answer that while having a directory listing will guarantee that they will spider your site, it gives no ranking advantage and you can use their new free submit or the site match program and get the same results.
On the other question that was brought up about Yahoo seeming to show referrers in site logs but not being able to find the listings I did find the answer - seems that a lot of surfers never clear their cookies - and if they still have an old cookie from before Yahoo switched over to their own database, Yahoo still has that cookie going to the old results database fed by Google - they havent paid any attention to it yet - and for some reason they've neglected to fix their old cookie direction script