My standard setup (on Debian stable/Lenny):
aptitude install unattended-upgrades
cat > /etc/apt/apt.conf.d/10periodic
APT::Periodic::Update-Package-Lists "1";
APT::Periodic::Unattended-Upgrade "1";
^D
cat >> /etc/apt/apt.conf.d/50unattended-upgrades
Unattended-Upgrade::Mail "example@example.com";
^D
It’s as easy as that. [More details in the Ubuntu Wiki]