How about making a package which enables/disables them, meaning no need to add commands to apt-get.
apt-get install universe
Will get a package from main which enables Universe. Removing the package removes Universe as a source.
Would this be too much of a support nightmare though, since main is fully supported, meaning that such packages must be fully supported (if they're not in main then there's no point having them since it doesn't make life easier), which could mean that Universe gets unwanted support requirements....
peterjs: I'm just pointing out that there is no cli equivalent to "Software Sources" in the text interface. Of course it's possible to do everything by hand, but not exactly user-friendly. User-friendliness is one of the stated goals of Ubuntu.
The views shown in this discussion are very disturbing to me, in the sense that it strengthens my feeling that Ubuntu and its community are losing touch with the UNIX philosophy. I'm all for making Linux friendlier on the desktop, but I use Ubuntu because it's UNIX with a nice UI, not the other way round.
Of course something so fundamental as /etc/apt/sources.list should have a CLI, of course the OP doesn't deserve a -1 for suggesting a CLI equivalent to a GUI tool, and of course that nano (or vi or emacs or whatever) is not a replacement for CLI. What's wrong with y'all?
I stand behind everything I said here for the usual Ubuntu ("desktop" Ubuntu). In the case of Ubuntu server, this is about ten times truer.
And for posterity, until this is resolved, here's how I'm going to solve it for now:
sudo perl -i -p -e 's#^(deb(-src)? http://archive.ubuntu.com/ubuntu/ [a-z]+ main restricted universe)$#\1 multiverse#g' /etc/apt/sources.list