Hi Mark,
even before talking to the host, I can tell you that I have these rules added in my wp-config.php especially to fix the IP problem (comments were registered as if they were all posted by my IP).
// ** bw 2012-08-12 wordpress reverse proxy x-forwarded-for ip fix ** //
if(isset($_SERVER['HTTP_X_FORWARDED_FOR'])) {
$xffaddrs = explode(',',$_SERVER['HTTP_X_FORWARDED_FOR']);
$_SERVER['REMOTE_ADDR'] = $xffaddrs[0];
}
Let me know if this helps.
Regards.