I was getting quite good ranking for one of my sites in both Y and G for my main keyword (within the first 3). About 4 months ago I found that my site disappeared from Y and luckily it was still ranking quite good in G. What the problem is that the site has joined a link building campaign which requires me to put some outbound links (which are unrelated to my site) from my site. I suspect that it was the problem and I took all the links away. After emailing Y for about 10 times in these 4 months. My site came up again in this few days and it gets the original ranking in Y for my main keywords. So, what I want to say is that. Though it may be just a kind of reversed luck to be caught by Y (Some people said that the chance was just the same as winning a lottery), you will be caught if someone report your site. It can be dangerous if your competitors are targeting at you. So, if it is possible, avoiding joining any link building programs which require you to put outbound links in your site. There are a lot of such programs, especially the Free ones, you will need to put a few hosted pages with the outbound links....
That's why you should always pay attention what sites you link to. You're lucky Google haven't penalized your site long before Yahoo did.
Yes I think I was lucky I was not caught by G. BTW, I have also sent you zexy a PM asking a question. Please reply after you receive. Thanks!