I have observed that many of the website including mine has loosed their indexing drastically in Yahoo! It looks like Yahoo! has refreshes their index table but they have not mentioned about it in their weather report. As per Yahoo! weather report last Search Update was performed in the start of April 2007. Also my ranking in Yahoo! is tweaking on daily basis. Which shows that something is going on at their side.
My site lost it's good position with the last update last month. I waited a few weeks, but none came back. I think I was filtered because of too many pages (I use a translator script, and had 26,000 pages in 8 languages) So I removed all but the 10 main pages, waited 2 more weeks, still way down in the serps. Then, I did a test with "yahoo search submit basic $49" If they accept your page (they did mine) Then it will get re-indexed every 7 days. (not every 3 months) My main page re-appeared high up in the serps ranking (yippie!) in 4 days after I applied. Now I just paid for my 3 other top paying pages as well (when and if they also come back, I will be able to pay for the whole deal $196 in just one day of lost sales) The main point here is that you can modify your page/site, and see results every 7 days)