I asked the same question a while ago. Opinion is mixed as to whether paid submission to Yahoo is worth it. General consensus is that no-one really knows for sure, so my advice is to take advantage of the strong dollar and go for it. It certainly can't do any harm, and directories don't come much more reputable than Yahoo.

BUT, it should only form part of a general otimisation campaign - other areas that are just as important are publishing frequently updated and relevant content, on-page SEO, general link building, etc.

(In my short time working on SEO, I've come to the conclusion that whatever question you ask, chances are you'll eventually receive 2 completely conflicting answers. I think a lot of what makes a site successful is just common sense, providing quality content that SE's WANT to index, and steering clear of deliberate attempts to spam the search engines - just my 2c).

John