I would give it a few days before getting concerned. It's happened to me and then I was thrown back in a day or two later.
Let me guess..your rankings are better in yahoo compared to google? Yahoos quality of sites is much worse then google and msn. Yahoo also loves those blackhat sites.
I am having problems with Yahoo as well. I am indexed but nowhere in the SERPs. My pages are now starting to fall off as well. I have 270 pages and all were indexed. This morning was down to 190. I was ranking between 5-50 in the SERPs for various terms. That was a week and a half ago. I have lost about 200 visitors a day and nearly $150/day in income. Yahoo accounted for half of my traffic. Sad part is I too don't know what I've done wrong.
Yahoo is is sending me a decent bit of traffic, but my rankings are all over the place. One day I'll be #22, one day I'll be #3, next I'm #65, now I'm back at #5.. I wonder if they randomize the results on purpose.. because I don't know that I've done anything on the site to cause such drastic random jumps.
The problem is that the people using yahoo are usually the ones looking to make a purchase, at least that is what I have been seening.
I run enough sites for that statement to not apply. I have sites in the Yahoo index that perform well, and these tend to be the sites that I have actually taken the time to write content for and optimise in a fairly "calm" way. These sites (There are 2 in particular) do not rank in Google. They are in what is generally known as "the sandbox". I have other sites that perform extremly well in both, and some that don't perform at all in Yahoo. But this is to a certian extent beside the point. We aren't talking about how well yahoo or google treat US, we are looking at the quality of the results they produce. Google's results right now are shit. Yahoo's are better.