My WordPress installation has the same problem. I had been using the admin account for posting. I set up a 'flickr' account to test posting via Flickr.com, and this worked fine, but when logged in as 'admin' I was not able to change the author of the items posted from Flickr. The admin account did not appear in the "post author" pulldown menu.
Tonight I renamed the admin user (via SQL, directly in the database), and this made the problem worse. As described above, now every post I create is attributed to the Flickr user.
There is *no* overlap in name or nickname here.
The problem appears to be in the wp_usermeta table. At least in my case, user_id 1 (the former admin user) has no row for meta-key='wp_user_level'.
The code that draws the Post Author pull-down calls a function
get_editable_user_ids(), which runs this query:
SELECT user_id FROM wp_usermeta WHERE meta_key = 'wp_user_level' AND meta_value != '0'
My user ID does not appear in the results set, because it has no wp_user_level row.
Probably the only fix is for me to create a new admin user and migrate all my posts to it, and simply abandon the current admin user. What a pain.
Theoretically I could add a row for user_id 1 to give it a wp_user_level in this table, but I don't know what appropriate values for the rest of the columns are.