http://docs.fedoraproject.org/en-US/Fedora/18/html/Installation_Guide/s1-netboot-pxe-config.html cp /boot/grub/splash.xpm.gz /var/lib/tftpboot/pxelinux/splash.xpm.gz Fedora 18 x86_64 no longer has /boot/grub/splash.xpm.gz . GRUB2 does not seems to have such file. (I do not yet know if it matters to omit this file.)
That line has been removed from the source and won't appear in the next release. I've also removed references to the grub splash image in other sections. Using a splash image is completely optional, and won't affect function, only aesthetics.
These changes have been published, closing.