At the time when /etc/rc.d/rc.sysinit tries to mount the shm-fs, my /var filesystem is not mounted yet and the needed mountpoint /var/shm is missing. That's why /var/shm will not be mounted here. Nothing uses shared memory before the normal filesystem-mountings, and so I have made an entry for /var/shm in /etc/fstab.
This is changed in the latest initscripts to use /dev/shm.