Debian! (Score 1)

If you can provide access to an archive of Debian GNU/Linux then your users can regularly upgrade their installations simply through the command "apt-get dist-upgrade". This will only upgrade just those parts of Linux which they currently have installed, and can easily be set up to run from a cron entry as well. Of course if you don't have any control you probably can't enforce a particular Linux distribution either! If you can select a particular distribution though Debian is definitely worth consideration because of these powerful network update features.