Update: 2nd December 2015
The instructions at the start of this thread are outdated.
Check out the code and install:
cd /usr/local
git clone https://github.com/letsencrypt/letsencrypt
cd letsencrypt/
./letsencrypt-auto
Get a cert:
/root/.local/share/letsencrypt/bin/letsencrypt certonly --webroot \
--webroot-path /var/www/example --renew-by-default --email \
example@example.org --text --agree-tos --agree-dev-preview -d \
site.example.org -d www.site.example.org