Comment 5 for bug 1037727

I verified by getting an image (amd64 20121210), mounting it loopback, modifying /etc/resolv.conf then:

export DEBIAN_FRONTEND=noninteractive
echo "deb $mirror $rel-proposed main universe" > "/etc/apt/sources.list.d/$rel-proposed.list"
{
apt-get update --assume-yes --quiet &&
apt-get install cloud-init --assume-yes --quiet --option Dpkg::Options::=--force-confold
} </dev/null

Launched an instance of that.
Then:
$ ec2metadata --availability-zone
nova
$ grep "^deb .*ubuntu.*$(lsb_release -sc) main" /etc/apt/sources.list
deb http://nova.clouds.archive.ubuntu.com/ubuntu/ precise main
$ sed -n '/system_info/,$p' /etc/cloud/cloud.cfg
system_info:
   package_mirrors:
     - arches: [i386, amd64]
       failsafe:
         primary: http://archive.ubuntu.com/ubuntu
         security: http://security.ubuntu.com/ubuntu
       search:
         primary:
           - http://%(ec2_region)s.ec2.archive.ubuntu.com/ubuntu/
           - http://%(availability_zone)s.clouds.archive.ubuntu.com/ubuntu/
         security: []
     - arches: [armhf, armel, default]
       failsafe:
         primary: http://ports.ubuntu.com/ubuntu-ports
         security: http://ports.ubuntu.com/ubuntu-ports