Have been reading reports on this for quite a good while, and figured it would make a good entry about what may be occuring. It seems that many people are complaining that Yahoo is dropping sites at a relatively odd rate. Some people are experiencing seeing their sites ranking in the top position only to be dropped the next day, and then back again the next day. What strikes me as odd is that two things seem to be happening to people. 1. The site or entire ip address range gets dropped permanently. 2. The site is in a state of flux, where they rank one day the next they don't.
While some of this can be attributed to test Yahoo is doing, such as Yahoo Local and Geo Targeting. There are many unanswered questions regarding this. There is a thread over at SEW forums where the member bbott, has observed Yahoo dropping his site one day and the next bringing it back. As well as an another member saying he has observed "clusters of sites drop and re-appear", and another report about an entire ISP having all 1500 urls dropped from the index for no reason.
There is another thread followed up at WebmasterWorld with some good discussion about people experiencing the same thing. There is discussion about this at Cre8site Forums here and here, SEOchat here and here, Highrankings here, Digitalpoint here. Some ideas that have been considered:
1. Yahoo uses more than One Index, maybe Two? 2. Harsh Penalties for Duplicate Content or another factor 3. Algorithm Changes 4. Local Search and Geo Targeting 5. Sketchy Guidelines
From the research conducted above, its seems one thing that most people are observing is that once a site is gone from yahoo, it might come back in a matter of a couple weeks, and that very small thing set off the Yahoo Filter.
These can be everything from link in the bottom nav, spammy content, to one that is very common interlinking between sites (big no no!). Things webmasters may not even be aware that could affect their rankings in Yahoo, often do. If you are concerned about why your site may have been dropped for a particular reason, I recommend giving it some patience. Look over your content, particularly the homepage for anything that might look potentially threatening to Yahoo, as well your linking network. Change it, if your site does not return. Second, send Yahoo some feedback about this. Locate the "feedback" link at the bottom of a search page. Secondly run ranking reports for Yahoo and look for patterns. Look at the details.
If anyone has any more feedback or suggestions regarding this, please post in the comments.