Based on the automatic updates article, the following configuration template, in conjunction with the unattended-upgrades package is meant to periodically update Debian to the recent stable distribution.
unattended-upgrades package with:aptitude install unattended-upgrades
/etc/apt and issue as root:svn export http://svn.grimore.org/configuration-templates/debian/apt/unattended-upgrades . --force
Issue:
unattended-upgrades --dry-run --debug
to check that the unattended upgrades are working.