/usr/X11R6/bin may be not in PATH in /etc/X11/xdm/Xsession. Try 1. Do server install and enable X login. 2. Install XFree86-xdm by hand. That is a separate anaconda bug. 3. Login as root in xdm. You will find some error messages in /root/.xsession-errors. That is because /usr/X11R6/bin is not in PATH.
*** This bug has been marked as a duplicate of 16648 ***