Trusted site should've worked but I won't argue the matter.

For extranet users that's where setting the AutoLogin IP range comes in. If the IP address is out of that range then they don't get the login box. See http://dev.gmss.org as an example. Here at home I get automatically logged in but for you it'll just be any normal website.

My solution:

Environment: Windows 2003 Server Standard Edition/SQL 2005 Standard Edition/DNN 04.09.04 Authentication: Active Directory for DNN 4 version 01.00.05 and builtin DNN

1. Add your DNN site to your user's Local Intranet Zone (Internet Explorer)(Internet Options/Security/Local Intranet/Sites)
2. Set "http://<Your Site>/DesktopModules/AuthenticationServices/ActiveDirectory/WindowsSignin.aspx?tabid=38" as your user's Home Page (Internet Explorer)(Internet Options/General/Home Page)
3. Ensure the IP range for your domain is correct

This allows the corporate office users to login automatically and the remote offices (not part of the domain) to use 'normal' DNN authenication. I can also login remotely using my domain login credentials (domain/username) so I get the best of both worlds.

Hopefully this will save someone else some stress :)