My experience with Yahoo mail is that there are tons of false positives on spam, and that none of the headers matter much... preference is given to a small number of whitelisted sending companies.
Anyways...
> "Yahoo doesn't consider a properly DKIM or Domainkeys email something that should not just end up in the spam folder"
Nor should they. There's plenty of actual spam that passes SPF and DKIM checks -- which brings us back to my original point (the amount of spam that I receive from Yahoo!). A message should not be treated as non-spam just because it passes those checks -- they are merely one factor to consider.
Of course not as the only criterion. But the messages in question were not spam, they were legitimate emails with DKIM, Domainkeys, SPF, long term non-spam IP address, etc.