Why do I keep getting a "delayed delivery notice" when I send email to Yahoo?

It turns out that this is a fairly large scale issue affecting many, many ISP's and hosting providers (MSN, Hotmail, Verizon, etc. are all companies where their users are having this trouble)  that are trying to send timely emails to Yahoo.  See here:
http://www.google.com/search?q=yahoo+email+delays

A common theory amongst Systems Administrators now is that Yahoo is getting ready to begin charging mail servers to send their customers "priority email."

Regardless, we are doing everything in our power to get our users email delivered to Yahoo in a timely manner.