Wildcard subdomains.

gistfile1.nginxconf

server {
        listen   80;

        server_name ~^(?<subdomain>.+?)\.domain\.com$;

        if (!-d /usr/share/nginx/$subdomain) {
                return http://www.domain.com/;
        }

        root /usr/share/nginx/$subdomain;

        index index.html index.htm index.php;

        location / {
                try_files $uri $uri/ /index.php?$query_string;
         }

        location ~ \.php$ {
                fastcgi_pass unix:/var/run/php5-fpm.sock;
                fastcgi_index index.php;
                include fastcgi_params;
                fastcgi_param SERVER_NAME $subdomain.domain.com;
                fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        }
}