I'm getting the similar behaviour to:
http://developer.redhat.com/bugzilla/show_bug.cgi?id=5001&BUGLIST=5001

  Here's were I am currently: three RH6.0 machines, all
upgraded to latest updates (XFree86-3.3.5-1.6.0) using "rpm
-F".  When I discovered the problem before shipping systems
(I was running as root until that point, so saw no problems
until final testing), I attempted to downgrade by mounting
the 6.0 CD and (in the RPMS directory) doing "rpm -Fhv X*".
This changed the error message given (to simply
"authentication failure") but did not fix the problem.  I
then redid the "rpm -Fhv X*" to reinstall the newer (if
broken RPMs).  I can now login at a console prompt as just
xyz, and do a "startx", but gdm still fails authentication.
  Ok, so what gives???  I'm curious why this was thought to
be a 5.2 problem.  Am I the only 6.0 user to experience it,
or am I the only one to upgrade???
  Don