This is straight from the mailing list with grateful thanks to Ryan Schmidt.
Sometimes I can be a bit slow so I hadn't even considered blocking access to the .svn folders on this and other sites I maintain. I have now! There are two solutions:
<Files ".svn">
Order allow,deny
Deny from all
</Files>
<DirectoryMatch "/\.svn/">
Order allow,deny
Deny from all
</DirectoryMatch>
and:
RedirectMatch 404 /\.svn(/|$)
I went for the latter which has the advantage of making it look like there are no .svn folders and it doesn't log any errors.