I just spent a day in tears trying to get gunicorn (python based) to work behind the ELB. I looks like it works fine behind nginx (fingers crossed). Make sure the elb health check is not going directly to the unicorn. It should go though nginx.
It is my understanding the the unicorn style is to get a request, handle it, then move on. The ELBs want to keep the connection open to improve efficiency. Nginx can handle many connections sticking around without any problem.