The Yahoo Search Japan blog announced they are updating their index now. Perhaps this is a sign that Yahoo in the US will update soon as well. I am pretty sure that both indexes are mostly separate for the most part, I am not sure if it works the same way at Google, but I think it does at Yahoo.
A WebmasterWorld thread has a single post about the update. One person who tracks Yahoo Search Japan said "we're experiencing big move on SERP."
The translated announcement reads:
Yahoo! In search, Yahoo! Inc. UEBUSACHIENJIN has developed the "Yahoo! Search Technology (YST)" has upgraded the search algorithm.
Recently we reported Index Update, unlike a significant upgrade and will affect the behavior of search engine algorithms.
The last US Yahoo Search update we reported was a while ago in March 2009.
Forum discussion at WebmasterWorld.