While the SVN Book talks about custom logging for SVN with Apache, the example format doesn’t include one of the most important pieces of information, the SVN repository being accessed! It took me quite a while to find this, so here it is. On SUSE Linux, add the following lines to mod_log_config.conf:

LogFormat "%{%Y-%m-%d %T}t %u@%h %>s repo:%{SVN-REPOS-NAME}e %{SVN-ACTION}e (%B Bytes in %T Sec)" svn

And the following line to default-server.conf:
CustomLog "/var/log/apache2/subversion_log" svn env=SVN-ACTION
Instead of the normal unreadable rubbish, this should emit lines similar to the following to /var/log/apache2/subversion_log:
2010-04-09 14:41:50 peternixon@ 200 repo:myproject checkout-or-export 
/ r5317 depth=infinity (875377105 Bytes in 217 Sec)