I’m having this same problem. We’ve got three administrators on the blog we just set up yesterday, and when we post, it’s not saying which one of us it was. I tried filling it out completely — even using fake info for the AIM number (I don’t use AIM), but it still doesn’t work.
One difference: we don’t even get a “Posted by” line. Our theme is Andreas09. Would that make a difference?
I found what was wrong in my case. It was the theme. I have to say I’m surprised that the theme affects the information displayed.