If you enable svn via apache and a SVNParentPath directive, you can add a SVNListParentPath On directive to your svn location to get a list of all repositories.
Your apache conf should look similar to this:
<Location /svn>
DAV svn
SVNParentPath "/net/svn/repositories"
# optional auth stuff
SVNListParentPath On # <--- Add this line to enable listing of all repos
</Location>