emclain:
I think you can make any change to the admin user, and it will fix this problem.
This happened to me as well. I added a new user and suddenly all the posts I had written while signed in as admin were listed as by the new user (also admin level), and my original admin user did not appear in the post author dropdown box.
I went to the original admin user and changed the nickname, and that added the account back into the dropdown list and got the post attributions to revert back to the appropriate author.
Hope this bug gets fixed. I thought I was going crazy!
