While it's been dead (or at least severely flaky) for some time I hadn't noticed until now as I generally use Stack Overflow from one laptop, and that has stayed logged in.
I only noticed the issue when I tried to log into the Careers site just now.
You've provided a very easy remedy, through the My Logins page, however this is only any use if you realise there's a problem while you are able to log in or are currently logged in.
If I'd cleared my cookies (which is very possible as I develop on this machine) I'd have been locked out, and I assume would have had to email a support email address to get back into my account.
Therefore it would be worth publicising the loss of myOpenID to Stack Overflow/Stack Exchange users while they are logged in, along with a note on how to avoid problems. You could target this at those users who have a myOpenID login registered, particularly those who have only myOpenID. These users would be able to fix their own accounts, and avoid generating a bunch of support traffic.
(It might also be worth removing it from the list on the login screen, or at least handling what happens when you click on it with a nice explanation rather than the current fail messages).