A number of people have posted to various mailing lists and made blog posts pointing to the Yahoo Mail blog post discussing recent problems Yahoo was having with mail. The general feeling seemed to be “AHA! That’s what is wrong!”
Unlike many of my peers, I do not think this explains the delivery problems senders have been seeing while attempting to deliver mail to Yahoo. The Yahoo mail blog article is talking about the Yahoo outgoing mailservers (smarthosts) for their non-webmail users. It is extremely unlikely that these are the same servers used for incoming email.
While I sympathize with everyone who had the AHA! moment and thought their delivery problems were being acknowledged and addressed by Yahoo! I do not think this is really what that blog post is saying.
I am hearing from people that Yahoo is aware of a problem with delayed incoming email, and they are working on fixing it. This does seem to be a broader problem than just bulk mailers, I am hearing from small and mid-size ISPs that they are having significant problems delivering email to Yahoo, too.
For more information about what Yahoo is doing to filter mail check out my previous post Greylisting: that which Yahoo! does not do.