Comment 1 for bug 18225

http://archive.ubuntu.com/ubuntu/dists/hoary/main/installer-i386/current/doc/manual/en/apcs01.html

It may be out of date, though, and I'm not sure that the country-code mirror
thing is actually preseedable yet. Current preseed/debconf (possibly post-Hoary)
allow you to tweak the seen flag separately.