I'm running my mod_perl server (apache 1.3.24 .. the work horse) behind my front end server (apache 2.0.36) using mod_proxy and noticed that it does not track the end users ip address properly in this case. I solved the problem by adding this as the first line of gforum.cgi:

if ($ENV{HTTP_X_FORWARDED_FOR}) { $ENV{REMOTE_ADDR} = $ENV{HTTP_X_FORWARDED_FOR} }

This works fine but there might be a better way to handle this in the long run. Just an FYI for future releases.