I just experienced this problem and fixed it with the workaround. I have WordPress 2.5.1. It must occur only under certain circumstances, but it seems to be still around, whether or not the causes are the same as before. I have only three site users: myself (admin) and two editors. As soon as I added the editors, I was only able to post as one of them, not as myself. “Changing” my status to Administrator worked. I can now post as myself, so I’m happy. It is a strange thing though …