I have set up 'unattended upgrades' on a 12.04 LTS machine. It's working quite fine with the exception of that it doesn't reboot even though all config files have been set to enable this.
/var/run/reboot-required exists and has the contents "\*\*\* System restart required \*\*\*"
Actually now that I'm thinking of it unattended upgrades is having some problems sending email to the recipient due to network/mail issues. This is the only (albeit remote) source of trouble I can think of.