When upgrading to filesystem-2011.12 there will be a conflict with /etc/mtab.
Install the package as follows:
pacman -S filesystem --force
It is strongly advised to avoid the --force or -f switch as it is not safe. However, in this particular case it is needed as deleting /etc/mtab manually would break pacman.
The reason for the conflict is that the file used to be generated at boot and hence not owned by any package. Now it is a symlink to /proc/self/mounts owned by filesystem. This change means that initscripts no longer requires write access to the rootfs (though other packages might).