The folder location /wp-admin is hardcoded into wordpress in multiple locations. It is pretty problematic to modify the location. WordPress will always go to /wp-admin. There are ways around this, but their are many steps.
You can forward a subdomain to www.mydomain.net/wp-admin but WordPress will still always go through www.mydomain.net/wp-admin.
Here is a reference if you really are interested in doing it…
http://lists.automattic.com/pipermail/wp-hackers/2010-July/thread.html#33458