You might also want to have a look on mod_simple_vhost. Here's the debian configuration example:
root@yourdomain:/etc/lighttpd# cat conf-enabled/10-simple-vhost.conf
## Simple name-based virtual hosting
##
## Documentation: /usr/share/doc/lighttpd-doc/simple-vhost.txt
## http://www.lighttpd.net/documentation/simple-vhost.html
server.modules += ( "mod_simple_vhost" )
## The document root of a virtual host isdocument-root =
## simple-vhost.server-root + $HTTP["host"] + simple-vhost.document-root
simple-vhost.server-root = "/srv/www/"
simple-vhost.document-root = "/htdocs/"
## the default host if no host is sent
simple-vhost.default-host = "yourdomain.com"
This would allow you to create any directory in /srv/www in form of "test.yourdomain.com", to activate this subdomain. Make sure you add the htdocs directory inside the directory.
But I guess you should check the DNS propagation at first, like Ben Quick mentioned.
Regards