Edho,

Thank you very much for the assistance. Here is what I used, and seems to be working perfectly:

server {
listen 80;

server_name ~^(?<user>.+)\.mydomain\.com$;

root /srv/www/users/$user/wp;

index index.php;

access_log /var/log/nginx/vhosts/$user.access.log;
error_log /var/log/nginx/vhosts/$user.error.log;

include /etc/nginx/excludes.conf;
include /etc/nginx/wordpress.conf;
include /etc/nginx/expires.conf;
}