I think owncloud would only need to have the supported architecture attribute modified in the PKGBUILD, but the main archlinux packages are pointing to older builds of owncloud that are not nearly as feature-complete as what is out there now.
I may just install it manually in the LAMP stack I've got, but this application looks like a pretty natural fit for ArchLinuxARM.
http://owncloud.org/
http://aur.archlinux.org/packages.php?O=0&K=owncloud