I've set up an rsync server on archlinux.org. This will let everybody keep their /usr/abs tree up to date, and it will also let you easily mirror the ftp site. I'm in need of download mirrors, so please, if anybody would like to volunteer, email me and I'll put you on the mirror page.
If you don't already have it, create the directory /usr/abs. Then to update your /usr/abs tree, you can do the following:
# cd /usr/abs && rsync -av archlinux.org::abs .