There is a fail-proof method to get Yahoo to index your site within 24 hours. All that you need to do is create an RSS feed, and then add it to your My Yahoo. Yahoo will then add your site towards the top of the spider's queue, and you will be indexed very quickly.
Also, it might be there but you're not noticing it. I've had problems recognizing it before, because it starts out with Mozilla and some numbers, and then says YahooSlurp. You might want to do CTRL-F and search your logs for Yahoo, to make sure that its not there. Otherwise, make an RSS feed and add it to your My Yahoo.