As of Midnight January 5th, 2010, Yahoo! Paid Inclusion is no more. Don't know what YSSP! or Paid Inclusion is? Don't worry then.
The Bad News!
With the alliance of Bingoo (Bing and Yahoo!), Yahoo! has shut down the paid inclusion of URLs in organic search results. If you participated in Paid Inclusion, you probably saw a decrease in organic search results (mainly because if you where already ranking well in Yahoo! you wouldn't have paid for special placement in Yahoo! in the first place).
The Good News?
If you are an SEO firm, you don't have to answer the question from clients, "Where would we rank if we weren't paying for position in Paid Inclusion."
Shortly, Bing will supply organic and paid search results for Yahoo!'s Search Engine interface. A move by Microsoft to take back some search market share from the behemoth we call Google. On the heals of Bingoo is Google Caffeine. Google Caffeine is billed as a boost in indexing for Googlebot but I am sure it will come with some personalized search enhancements.
SEO changes so quickly it is hard to keep up. That is why I love SEO!