When an Elastic Load Balancer handles a connection it sends it’s own (internal/private/10.x) address instead of the clients. It sends the clients along with the request as X-Forwarded-For. To log this you need to log X-Forwarded-For instead of the source IP.

Here are 2 links discussing the problem, the first covers a basic Apache & Tomcat setup, but the second one has a point about direct access getting not logged and has an Apache httpd specific solution.

http://blog.kenweiner.com/2009/09/amazon-elb-capturing-client-ip-address.html

http://blog.grahampoulter.com/2011/10/how-to-log-client-ip-from-apache-behind.html

Thanks @grahampoulter and @kweiner