Very simple, and hinted at in the Debian APT HOWTO. I think I used to use a different package for this, but I think I like the simplicity of this method better. After installing apt-show-versions, I made the following entry in root’s crontab:
@daily apt-get -q -q update 2> /dev/null && apt-show-versions -u
root gets mail if apt-show-versions generates output, indicating new packages are available. That mail is redirected to my user account on the system, which is then scooped up by gmail where I see it and act upon it. I could automate the entire update process, especially since the stable release’s updates are seldom and safe (ie: security fixes are backported), but I’d rather do it myself to be extra careful.