who shows (unknown) user logged-in: what's going on?

After reading Centimane's comment on /var/run/utmp and searching differently, I found this fedora forum thread, which mentioned the issue is provoked by a bug in GDM, which creates a bad entry in /var/run/utmp. Eventually I even found a bug report for it and another here.

It really seems like your unknown user is gdm and who somehow fail to interpret it.