Could you be more specific as to what you mean by "I guess I did something"?
Were you playing with anything or installing any modifications or apps right before this occurred?
I found a reference to your problem on SGI's open source page that talks about a known issue with XFree86 and devfs. See here for more info:
http://linux-xfs.sgi.com/projects/xf.../msg03069.html
Some other suggestions would be to check permissions and for troublesome tmp files. Both are described in this thread:
http://archive.lug.boulder.co.us/bym.../msg00197.html
Personally, I think it sounds like a PAM authentication problem. Unfortunately, I know very little about PAM. What does your /etc/pam.d/xserver file contain? Mine has:
[root@CY615927-A /root]# cat /etc/pam.d/xserver
#%PAM-1.0
auth sufficient /lib/security/pam_rootok.so
auth required /lib/security/pam_console.so
account required /lib/security/pam_permit.so
Compare that to what you have.