A zoneminder-svn packagebuild is in AUR
It is recommended that you NOT try to update an installation from a zoneminder AUR package prior to version 1.24.0. The default archlinux apache configuration has changed markedly. zoneminder-svn and future zoneminder 1.24.x packages will store events under /var rather than /home so fresh installation is highly recommended.
A complete preconfigured Arch Linux system, with zoneminder preinstalled is availible on the ZMLarch livecd.