Присоединяюсь к вопросу. Аналогичная ситуация, но proxy_buffering не включен.
FreeBSD 8.2, nginx 1.0.5, apache-itk 2.2.17

Конфиг:

user www www;
worker_processes 8;

error_log /var/log/nginx/error.log notice;

pid /var/run/nginx.pid;

events {
worker_connections 1024;
use kqueue;
}


http {
include mime.types;
default_type application/octet-stream;

access_log off;

sendfile on;
tcp_nopush on;

gzip on;
gzip_min_length 1000;
gzip_proxied any;
gzip_types text/plain text/xml application/xml application/x-javascript text/javascript text/css text/json;
gzip_disable "msie6";
gzip_comp_level 6;

proxy_cache_path /var/db/nginx levels=1:2 keys_zone=one:5000m inactive=30d max_size=5G;
proxy_cache_min_uses 1;
proxy_cache_valid 30m;
proxy_connect_timeout 75;
proxy_max_temp_file_size 16096m;

client_max_body_size 10240m;

limit_zone three $binary_remote_addr 10m;

server {

listen 80;
server_name fs.domain.ru;

location / {
proxy_pass http://127.0.0.1:8181;
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_cache off;
access_log off;
allow all;
}
}

}
}