simpler solution that does what the others have posted more succinctly:
sudo perl -p -i -n -e "s/# *deb-src/deb-src/" /etc/apt/sources.listKey distinctions: Perl has the -i inplace option which modifies files in place; I did not add a suffix for backup files because I didn't want the backup files to accidentally be treated as data files. And " *" deals with optional whitespace. "perl -p -n -e" is mostly the same as "sed -e", though watch out for greedy regex matching.
It has the same limitations the others do: it enables sources even if the original wasn't enabled (i.e. partners), and it doesn't work on /etc/apt/sources.list.d. The following will also process /etc/apt/sources.list.d* but makes a backup first.
(cd /etc/apt/; sudo tar cvf sources.list.tar sources.list sources.list.d); for i in /etc/apt/sources.list /etc/apt/sources.list.d/*; do sudo perl -p -i -n -e "s/# *deb-src/deb-src/" $i; done