I think that has something to do with members who had their accounts "fixed" so that they could change their avatars. All of their old posts seem to be turned into "guest" posts. I've seen the same happen to Captain_insano and a few others.
Note that I'm not a guest anywhere... that's cause my account is broken and I'm leaving it like that. =) Why would I want to change my awesome avatar anyway?
You are correct sir. It was/is to fix broken accounts which in turn break again. Had mine fixed 2 weeks ago and is in worse condition. Not only can I not change an avatar again, not vote on a poll, now my spell checker does not even work anymore.