I am trying m0n0wall + Windows 2008 server NPS (Radius) setup too, with out much success...
I used the guide her:
http://www.bunkerhollow.com/blogs/matt/archive/2008/06/04/configuring-server-2008-for-radius-authentication.aspxas a starting point, and have come as far as to getting "
A LDAP connection with domain controller <MY DC> for domain <MY DOMAIN> is established." in the eventlog...
m0n0wall captive portal is just not letting me thru the Captive Portal, but sends me to the Authentication error page, where $PORTAL_MESSAGE$ is empty...
How far did you or others get?