It sounds like they may have cleared their "Display Name" setting from
their user record in that site collection. That's the only way I know
of that would cause "Blank" created by/modified by metadata to be
associated to the items.

You could programmatically enumerate those items, and for the ones where
Created By/Modified by are blank, use the associated UserID to then go
back to the site collection users list  to find out who it is. I don't
think it would be that difficult to do. If you'd like, I'll try to
take a stab at creating something fairly crude but workable in a couple
of days, but first I want to ensure that specifically what you are
seeing, is in fact, just "Blank" created by/modified by

So, it looks like a need from SPS2003, WSS
V3?