After much investigation and messing about with my server setup, I have figured it out :) Now, my Ghost install runs on Ubuntu 12.04, with nginx 1.4.x. Here's the directive that I'm using:
[...]
location / {
alias /var/www/site;
proxy_pass http://localhost:2368/;
proxy_set_header Host $host;
proxy_buffering off;
}
[...]