Just that...

[root@adia:~]# cat /etc/mtab
/dev/hda1 / xfs rw 0 0
proc /proc proc rw 0 0
/dev/hda2 /usr xfs rw 0 0
/dev/hda6 /pub ext3 ro,noexec,noatime 0 0
/dev/hdb2 /pub/Images xfs rw,noexec,nosuid,noatime 0 0
/dev/hdb3 /pub/httpd xfs rw 0 0
tmpfs /dev/shm tmpfs rw,size=16m 0 0
usbfs /proc/bus/usb usbfs rw 0 0
/dev/hda5 /home ext3 rw,nosuid 0 0
/dev/hdb1 /var xfs rw 0 0
sysfs /sys sysfs rw 0 0
[root@adia:~]# cp /etc/mtab /var/lib/misc/mtab
[root@adia:~]# rm /etc/mtab
[root@adia:~]# ln -s /var/lib/misc/mtab /etc/mtab
[root@adia:~]# cat /etc/mtab
/dev/hda1 / xfs rw 0 0
proc /proc proc rw 0 0
/dev/hda2 /usr xfs rw 0 0
/dev/hda6 /pub ext3 ro,noexec,noatime 0 0
/dev/hdb2 /pub/Images xfs rw,noexec,nosuid,noatime 0 0
/dev/hdb3 /pub/httpd xfs rw 0 0
tmpfs /dev/shm tmpfs rw,size=16m 0 0
usbfs /proc/bus/usb usbfs rw 0 0
/dev/hda5 /home ext3 rw,nosuid 0 0
/dev/hdb1 /var xfs rw 0 0
sysfs /sys sysfs rw 0 0
[root@adia:~]# umount /sys
[root@adia:~]# cat /etc/mtab
/dev/hda1 / xfs rw 0 0
proc /proc proc rw 0 0
/dev/hda2 /usr xfs rw 0 0
/dev/hda6 /pub ext3 ro,noexec,noatime 0 0
/dev/hdb2 /pub/Images xfs rw,noexec,nosuid,noatime 0 0
/dev/hdb3 /pub/httpd xfs rw 0 0
tmpfs /dev/shm tmpfs rw,size=16m 0 0
usbfs /proc/bus/usb usbfs rw 0 0
/dev/hda5 /home ext3 rw,nosuid 0 0
/dev/hdb1 /var xfs rw 0 0
sysfs /sys sysfs rw 0 0
[root@adia:~]# mount /sys
[root@adia:~]# rm /etc/mtab
[root@adia:~]# cp /var/lib/misc/mtab /etc/mtab
[root@adia:~]# cat /etc/mtab
/dev/hda1 / xfs rw 0 0
proc /proc proc rw 0 0
/dev/hda2 /usr xfs rw 0 0
/dev/hda6 /pub ext3 ro,noexec,noatime 0 0
/dev/hdb2 /pub/Images xfs rw,noexec,nosuid,noatime 0 0
/dev/hdb3 /pub/httpd xfs rw 0 0
tmpfs /dev/shm tmpfs rw,size=16m 0 0
usbfs /proc/bus/usb usbfs rw 0 0
/dev/hda5 /home ext3 rw,nosuid 0 0
/dev/hdb1 /var xfs rw 0 0
sysfs /sys sysfs rw 0 0
[root@adia:~]# umount /sys
[root@adia:~]# cat /etc/mtab
/dev/hda1 / xfs rw 0 0
proc /proc proc rw 0 0
/dev/hda2 /usr xfs rw 0 0
/dev/hda6 /pub ext3 ro,noexec,noatime 0 0
/dev/hdb2 /pub/Images xfs rw,noexec,nosuid,noatime 0 0
/dev/hdb3 /pub/httpd xfs rw 0 0
tmpfs /dev/shm tmpfs rw,size=16m 0 0
usbfs /proc/bus/usb usbfs rw 0 0
/dev/hda5 /home ext3 rw,nosuid 0 0
/dev/hdb1 /var xfs rw 0 0
[root@adia:~]# mount /sys

Why would mount not write to mtab if it were a sym link?

Well, the /etc/mtab is hard coded in glibc, but its open source so I changed it to point to the place on /var I wanted it to go, recompiled, recompiled util-linux (mount), and it works fine. I left /etc/mtab as a symlink in case other apps want to read it.