Despite being successfully ranked in google, our site is not doing well in Yahoo. We have 8,000 pages indexed in Yahoo. None of these pages rank what so ever. We did rank for sometime until April when Yahoo did an update. We ran a test of a group of site URLs against the Yahoo index using the site explorer API and found that 95% of them were found in the index but failed to return a page title, they just returned the URL. Also there is no description or cache version. We don’t have this problem with Google and there are no differences in the title tags between the two groups. What might be causing this?
Yahoo penalizes sites manually if they feel they "shouldn't" be in the index, and that feeling is quite subjective. That may be what happened to you, or it might be something else. I know sometime around April, they began to penalize sites that had too many "suspicious looking" links to them, too.
One possibility albeit unlikely... you opted into the Yahoo paid inclusion program but left the title field blank? Perhaps you are a really large company and your Yahoo paid inclusion campaign has been forgotten about.
Yes, it's a stretch but I have seen odd things like this happen before with wasted dollars being thrown at old Marketing campaigns ran by long gone employees.