I too noticed that a few recently asked questions cached in google are now deleted on yahoo´s system. Mark, Domain Keys seemed to increase the "chances" of going through.
After installing Domain Keys we got a lot of messages (compared to none before) to go through, but then all of a sudden they started being sporadically blocked again.
There appears to have been an incident involving capacity issues within our delivery infrastructure.
You may retry sending at a later time when you see this message.
However please note that emails from the mail server(s) you are using may also have recently become deprioritized due to potential issues with its mailings.
I submitted 2 tickets with yahoo over the past week and half and still have not heard anything back from them. I did that on Wednesday, but have not yet received a reply.
I actually submitted the whitelist form several months ago (in May) and was told I would now have delivery to the inbox. On a side note, I also noticed that my Emails per day received on my Yahoo account are much less.
(They have a high incentive because work is on a first-come-first serve basis.