OK, I finally got some error message into logs:

2011/04/26 13:44:39 [error] 5727#0: *1 upstream timed out (110: Connection timed out) while connecting to upstream, client: 206.53.59.32, server: sandbox.domain.com, request: "GET /info.php HTTP/1.1", upstream: "fastcgi://192.168.0.10:9000", host: "sandbox.domain.com"
2011/04/26 13:44:57 [error] 5727#0: *1 connect() failed (113: No route to host) while connecting to upstream, client: 206.53.59.32, server: sandbox.domain.com, request: "GET /info.php?=PHPE9568F35-D428-11d2-A769-00AA001ACF42 HTTP/1.1", upstream: "fastcgi://192.168.0.10:9000", host: "sandbox.domain.com", referrer: "http://sandbox.domain.com/info.php"

/info.php?=PHPE9568F35-D428-11d2-A769-00AA001ACF42 is the php image we see into phpinfo regular page.