Reports on the Mailop mailing list suggest that the issue started around 7:00 am US central time on Sunday, March 20th.
Update: As of 1:50 pm US central time, I see that some mail must be getting through for some folks. My primary Yahoo seed mailbox is populated with a new message every few minutes or so. But volume is a lot lower than usual and mail is still backed up on my sending server.
Final update: Multiple sites are reporting that as of about 2:00 pm US central time, Yahoo seems to be accepting all mail again. My queues are empty and all of my earlier-sent test messages have now been delivered to my Yahoo seed mailbox.