Although Portage takes care of configuration files very well, no system can have
enough protection from human errors. Versioning configuration files under /etc
is a nice solution to this. If you do a mistake and delete some file or any
other silliness that might happen, you just revert to a former revision and
solve the problem.

This can be achieved easily:

# First create a subversion repository:
$ svnadmin create /root/svn
# Then import /etc to the repository:
$ svn import /etc file:///root/svn -m 'Initial import'
# Checkout the project:
$ rm -fr /etc
$ svn ci file:///root/svn/etc

Now change some configuration files and do svn status . When you change your
configuration files, you can commit them to the repository:

cd /etc
svn ci -m 'Changed MAKEOPTS to -j2 in /etc/make.conf'

Linux rulez!