Valentin,

Ok, well I am almost there. The problem is since we need to define the map block outside of server, the $user variable is not defined yet. I.E.

http {
# $user is not defined yet, it comes from inside server {}
map $user $user_fastcgi_port {
default 9000;
justin 9001;
simon 9002;
}
}

server {
# Set's $user
server_name ~^(?<user>.+)\.my\.domain\.com$;
}