I've been running WordPress on Apache with CloudFlare for a while with no problems (using their Apache plugin which respects their CF-Connecting-IP header).
I just installed nginx to try and get a performance boost, and so far that's working fine. I've even added 'set_real_ip_from' and 'real_ip_header' to nginx.conf as recommended by CloudFlare. The Apache logs continue to show the correct IP, as does one of the WP statistics plugins I use.
Commenter IPs, however, are not correct and all show as the IP of my server. Any ideas on what could be causing this?